I think the issue with these champions has less to do with the fact that their sustain is manaless, and more to do with the fact that their sustain is just poorly gated, period. At the end of the day, sustain is the negation of damage, which in lane means it is usually also the negation of plays made by one's opponent: because of this, in-lane sustain should represent the product of an equal or better play, namely by landing a difficult ability, putting oneself at risk, etc. Under no circumstances should healing just come out of thin air, and the absolute worst form of sustain one can have is to be able to just hang back and press a button at no meaningful cost or cooldown (which is unfortunately all too common on quite a few support kits still). With champions like Aatrox, Akali, or Vlad, the issue has mostly been that their healing comes from either just autoattacking at melee range or using short-ranged spells: while this could be more justified in a lane heavy with ranged harass or poke, such as mid or bot, it is much less so in top lane, where everyone is short-ranged, but not everyone is manaless (most top laners have healing, shielding or both, however). As a result, their sustain tends to feel unfair.
What bugs me about requests to tack mana onto manaless champions, however, is that mana is only a means to an end, not an end in itself: ultimately, the only purpose to mana is to act as a resource gate over an extended period of time, mainly so that champions in lane are forced to back if their kits are a) too safe, or b) abusive in lane. Issue A doesn't apply here, or at least shouldn't, because these champions are expected to expose themselves to attacks (and if they don't, that's an issue that needs to be resolved), and issue B shouldn't apply because abusive kits should be answered at their source, rather than with band-aids. Effectively, the bottom line is that if manaless champions don't offer enough counterplay in lane, then chances are that their problems come from dysfunction in their kits or individual abilities, rather than their resource system.
One thing I also think is responsible for making sustain champions feel unhealthy is how their sustain interacts with scaling: when a champion has too much static health, there are generally at least ways to play around that via poke, but when a champion has too much sustain (and typically those same champions are fairly durable as well), the only way to counter that is through burst. Because burst tends to be all-or-nothing, and is itself heavily dependent upon scaling, this means that if you don't have access to good burst, and you're dealing with an overly fed sustain champion, you're screwed. Beyond just champions with innate sustain, this is also a problem with marksmen whenever they have life steal or any kind of persistent healing or shielding, because it puts bursty champions on a razor's edge of balance where even slightly insufficient burst can be severely mitigated in very short order. Excessive stat differences tend to mess balance up anyways, but it's particularly bad when it comes to power with low downtime, as is the case for most kinds of self-healing.
seem extremely hard to balance appropriately. Does anyone remember early Aatrox? His base stats were entirely to high for his Kit so they nerfed his base stats multiple times until he was quite frankly... to weak to even lane. He had a few reworks since then, but it's lookin like he maybe getting more changes in the future. When you have a "Carry" level of damage in a kit that is entirely built around sustain some extremely silly things can happen especially if they snowball, but mainly late game these champions at full build are pretty godly (look at you Vlad). So Riot in the future could we avoid Mana-less (or just resource-less champs like Vlad) with high level sustains kits? Give them a reason to have to go back to base, give them a reason to consider not spamming spells, give them some Mana to gate their power/lane pressure!
for a reason in lane. Mana champions cannot spam this skills in lane to CS and harass they will go oom. So you have to choose, personally i use my mana to harass and farm under tower,and not really to cs minions unless i have too. Champions without mana like Vlad just sit in lane spam skills on minions and actually gain HP and resources for laning instead of your skills actually costing you something like Syndra would.