Now here's where comparisons can't be made. There are many different win conditions for different champions and many different types of enchanters. All of which have different win conditions / success cases / value. So yeah you can make the comparison with every other enchanter and it will be true because all enchanters are different and provide different things. If they didn't than it would just be an arms race, "which champion is the best". I can't say that Thresh is a better support than Soraka because they both have different jobs. The only reason I mentioned Kayle's healing potential and how it is equal to the likes of Soraka, Sona, Nami, is because I assumed you thought she couldn't heal as much. Comparing heals is fine though, comparing the entire champion is not because they got different strengths.
Sona will never:
- Provide the same amount of CC as Nami
- Provide the same amount of peeling as Lulu
- Provide the same amount of engage as Rakan
- Provide the same amount of zone control as Ivern
- Provide the same amount of invulnerability as Kayle
- Provide the same amount of disengage as Janna
- Provide the same amount of healing as Soraka
- Provide the same amount of catch potential as Blitzcrank
However Sona is viable as a support because she has other strengths. Some strengths like damage reduction, synergizing the most with death ball compositions, and her ability to protect her whole team albeit less than single target protection other supports have. Making these kinds of comparisons are pointless. Once again, I only brought up Kayle's healing to show that she does in fact have good heals and because I assumed you thought she didn't. I also listed her stuff in comparison to others and showed that she does in fact have utility worthy of being a support. I didn't bother to address the aspects of her kit because those are unique to her and Kayle has her own unique stuff.
The comparison I made is justified because I am comparing the champions ability to enable their team to do whatever. Alistar, Nami, Kayle, Thresh, Rakan, all have ways to enable their team through heals, shields, engage, CC, and can work off of low income and experience because of the nature of their kits. Lux, Ornn, Nunu, Taliyah, are all unreliable and require gold and experience to be successful which is not the norm for supports, they also do a poor job of enabling their teams. The comparison you made is not legitimate because you're comparing things that Kayle doesn't even have, Kayle has her own strengths, Sona has hers.
I also put the things Kayle has in comparison to other enchanter types:
- Nami: heal, slows, hard CC, ms buffs
- Sona, heal, shield, MS buff, hard CC, damage reduction, slows, extra damage
- Lulu: polymorph, shield, A/S buff, heal, knock back, slow, MS buff
- Soraka: heal, slows, silence, root
- Kayle: heal, MS buff, invulnerability, slows, resist shred, "extra damage"
So yeah. Probably can't get stats on how much resists shredded per game and how much that increased damage. Can't get stats on how much damage negated by invulnerability. Can't get stats on how much MS was given to teammates either.
I build Kayle with Rylai's usually (she has diverse builds) which means I got a permanent 20% slow in an AoE which is great. Kayle doesn't work like all other enchantresses because she needs to build value from auto attacking to apply her passive and gain value from her builds. Her heals matches other heal type champions so she's viable in this department. What separates her is that she has access to permanent slow, armor shred and invulnerability, and she does a good job at all of these. Kayle as a support means that she doesn't just ultimate herself (which is what usually happens) and instead enables her teammate to make more risque plays that they couldn't have before without invulnerability. That assassin, diver, marksmen, or whatever can now go further than before because they're not taking damage. Also invulnerability means you basically win all life steal trades because unless you heal on-hit you're not life stealing from an invulnerable target, denies all types of bursts / executes.
Nunu's potential comes from his ability to shut down the enemy jungler, lock them out of experience and making early plays. So you're wasting that potential as a support because you're only playing with 2 abilities now. Heck you don't even use your passive now because you shouldn't be auto attacking that much as a support, unless you're constantly pushing the wave for whatever reason and prepared to get ganked with the minimal CC you provide. So you're wasting like 40% of his kit really, even more than that because of the passives and stats he gains on his Q.
Once again Nunu has no tank steroids in his kit or hard CC to lock down people (a standard for tankier supports), so he is only tankier than other enchanters if he builds tankier. You're not doing your job if you die easily. Alistar, Braum, Leona, Taric, all do their job because they have innate tank steroids in their kits that allows them to soak up damage and survive. Nunu unless ahead is squishy because of his support income tank build and will be easy to walk over. It's like if you die as any other support that's not tanky, you're not doing much and the enemy team will just continue to run your team down. So that Soraka that got caught out, her death doesn't mean that much because it didn't require that many resources, neither does killing a Nunu that doesn't have gold income.
If you're picking Nunu for chase down than it's comparable to almost all other supports because all other supports either have slows + MS buffs or hard CC. Nunu does have strong zone control with his ultimate but it gets easily shut down by CC and, once again, if you haven't invested in tank stats you're not gonna be surviving long if you get caught. Losing objectives like baron or dragon are very much a possibility because you're maxing Q last (hopefully as a support), so unless you're really coordinated with your jungler there is a chance of losing an objective. Also 300 true damage is pretty lackluster as the game progresses against objectives that have thousands of health (damage dealers will deal more with AAs / spells).
You are only seeing Kayle's damage potential because of her item builds. However if that is the case than I'm not losing potential at all. Kayle has 60 (+30%AP) on-hit and it also splashes to enemies. Well she also has a 240(+45%AP) heal w/ 30%(+7% / 100AP) MS buff as well. So I see no problem with choosing to build around this ability over her E. Sure damage potential is lost but you trade that for supportive potential while still using all of your kit. PQWER, are all still used when you play her support in comparison to Nunu who for most of the game only uses W/E. Kayle as a support doesn't need that much gold, doesn't need that much experience, and enables her team in a way no other support can. So no potential is lost.
Gragas can play tank or he can go AP because of his ratios, he is not throwing out half of his kits potential by choosing either build. Or you could look at Trundle, you can go for the fighter juggernaut build and take advantage of his Q and W or play him support and work around his E a lot more as shown in LCS. Look at Vi where most people go for a Tri-Force/E centric build when she scales best with AD (her W and Q), it's not a straight up loss of potential, it's potential moved else where in a kit. So changing the item build doesn't ruin the potential of certain champions. Kayle included because she can benefit and gain value from W and playing supportive to enable her team.
I compared Kayle to off-meta supports like Taliyah, Nunu, etc, because I wanted to show the distinction in their ways to "support" in comparison to Kayle's and how they don't work whereas Kayle does although she's not meta. Those champions don't really support or have the abilities to effectively support whereas Kayle does. Kayle, Thresh, Braum, Sona, all have ways to enable their teams without gold or experience. Nunu, Taliyah, Ornn, Lux need gold and experience while also being pretty unreliable. I'd like to point out that the only off-support champion you went deeper into was Nunu, saying he is tankier, and has more zone control / chase potential, I addressed all these points and showed why he doesn't and how he is just inefficient as a support.
The main point of my post is "This is one reason why I enjoy Kayle support more than other heal/shield type supports." so this whole discussion is a huge tangent lol.