as far as the windows of weakness and cooldowns, thats exactly how they kite. Blow cooldowns, leave combat. ReEnter Combat. Sure there is a window where they cant deal dmg, but retaliation during that window is nearly impossible. You cant deal dmg back to them. And then there is the ever present disengage spell. Such as syndra's.
Contrast that with the adc, whos window is open while they are dealing dmg and its open for as long as they deal dmg, and they wont deal appreciable dmg unless they are constantly dealing dmg. If you wont admit that adcs have stronger weaknesses than mages in the current state of the game, then you are not worth talking to. I just played against a 4 item mundo with my 4 item ashe. His auto attacks dealt more than twice the dmg of my auto attacks with infinity edge, mortal reminder, zeal item, and BORK! I was dealing 80 dmg per auto. He was dealing over 180. Im not saying that syndra would have been able to kill him. She wouldnt have. But I do think it would have been 10X more difficult for mundo to start chaining cleavers and slows on her because she doesnt have to sacrifice any of her dps to kite or position.
In fact, syndra should really just position so that she can ignore the mundo and thats what mages are really good at. Having the range and burst dmg such that it is more effective to ignore the tanks as much as possible and target squishies. Thats not a window of opportunity. Thats a lack of interaction. They also clear waves before you can get under tower. I admit to carrying games through that dynamic alone with viktor. All i did was clear waves before enemy could siege and they stopped grouping. Which allowed my team to catch up and win. Thats a denial of interaction.
As far as your complaint that adcs dont have windows. Well, they are dps champions. That means that their dmg is broad with respect to time. However, I wouldnt be opposed to giving my immobile adc's something like ashe q, which is re balanced to give "windows" of increased or decreased dps. As long as their ability to dps is net INCREASED. Remember my game against mundo? How is it that a defensive champion out dps an offensive champion under equal item constraint? And by a HUGE margin. It would have taken more than 30 auto attacks to kill him, but it took him only five instances of dmg to kill me.
Clearly mundo isnt overpowered. Im not saying that. I am saying that the things he does dont really make sense in this game and yea he takes way too long to kill. You want to argue adc is broken, but the ranged auto attacks have a very SENSIBLE role in the game. The only sense that mundo makes is that it is strictly impossible to kill him in 5-15 seconds and he will absolutely kill you in less than 5 seconds, if GIVEN the opportunity. That is, if you GIVE him that opportunity. He also facechecks bushes. Nice.
I think that the only thing that is good about being adc is that with some, getting fed makes it very easy and straightforward to end the game. Seems to me that every champion should be able to end the game with roughly equal facility. Mundo doesnt have a way of doing that nearly as well, i suppose. Thats why he is broken, overpowered, and still weak as hell, I suppose. That doesnt have a thing to do with the ADC class, however. It has to do with the fact that this is a siege game vs ranged objectives.
If you ask me, and ive been saying this for a long time, tanks need to have some ability to defend towers. Right now, its the mages defending towers via waveclear, adcs destroying towers, tanks facecheck everything, be it teamfights or bushes or unwarded areas (which does actually make sense), and supports use the little ghosties, deward shit, and frustrate their opponents. Also, Zyra does a fuck ton as support. Thats the meta. Its quite lame in some respects.
Im sorry tanks dont have a better role for you. In my opinion, they shouldnt be running forward trying to deal dmg. Now, they should be holding ground, dealing dmg. Let the bruisers run forward. The tanks could hold them back. That would be a pretty decent role. And secondarily they would have some waveclear to defend towers after a fight if they survive. Not primary waveclear like victor, but something. I wouldnt be opposed to that. But I am opposed to an unkillable monster running into my team with zero fucks given. Its balanced at the moment. Mundo clearly isnt overpowered, but its not good gameplay. Lethal dmg + unkillable defense is not a gameplay enhancing idea.