Youre correct.
Trying to support an autofilled ADC is the worst. They would be better off buying the sight stone and letting you farm. What will end up happening is they will over commit to nearly every opening. They dont understand skirmish/trade in bot lane. They see someone hurt, then go all in and dont understand how much grief that enemy support can give them and how they can actually die if they dont turn back. They dont understand why you didnt go in with them and believe it would have been an easy double if you did...
Of course I also see SOOooo... many (going to create the official autofill abv. now) atfl sups against me a sup main. My last match I was 2nd pick and got Bard, the opposing support go to pick 8th, and picked Taric into me... I dont recommend playing any tank support into Bard, not even Leona or Braum. They got Taric, and also bought a golden coin... Now Im not dissing the golden coin, its actually my sup item of choice on certain more tanky supports, and some squishy that both need HP and mana sustain. I feel as though Taric doesnt need that much mana. I fee its manageable on him, and that relic shield is a much better option overall. An opinion. Well they were destroyed.
Autofill cripples bot lane placements. Its the most complicated lane in the meta. I see tops come down to support, and pick their top main and not build a support item for gold income. They just play/build as if theyre top lane without taking cs.... they also clearly dont understand the dynamic of a duo lane, and are roughly 100% useless in mid game... for your team, theyre an incredibly useful bag of gold for the enemy team to pad their stats/wallets on.
Jungle mains are often worse than tops when autofilled to bot lane. Im not saying theyre bad players, but they see less laning time than other players so they tend to be slightly weaker on laning mechanics like freezing, or last hitting. Sadly they are always profoundly surprised that that support they normally stomp on in a gank is giving them such a rough time in lane.
Mid laners if they get support are often much like top, they will just build as if they were mid lane... I see this a lot. They get thrown off pretty hard because their powerspikes from itemization dont come because of the lowered gold income. They also often misinterpret their positioning and durability in a duo lane environment. Its rare for them to build a sightstone and they get the shit ganked out of their lane because A: our team doesnt want two carries to emerge from bot lane, and B: its easier to get at a lane that doesnt invest in vision.
This is all on top of the fact that bot lane is the 4 man gangbang run a train on it lane target prime. If you have people who are even slightly getting behind, that lane is over and 2/5 of your team is about to be shit on and you will be able to smell the dumpster-fire for the rest of the match as 40% of your team enters mid game behind or useless.
Having autofilled people stink up their lane and feed sucks anywhere, the increased problem with it botlane is that it tends to affect the adjacent player a lot. That one autofilled trash player bot lane doesnt just feed one enemy laner that becomes a monster, they usually feed two.
Autofill sucks, it has lowered the quality of many of my matches and thereby the quality of the game for me. Maybe like dynamic que they will drop this shit next season for something better like "gain +2 LP on wins in matches where you were autofilled." This will ensure that some portion of the playerbase has autofill enabled, and people who can field multiple roles and win deserve to climb as they are obviously more skilled than those who cant.