In short, all of them. Agree with the post above that the cruise control -- in either adaptive or non-adaptive mode -- is the worst offender. Due to its limitations, I use the adaptive mode in only very limited circumstances. I use the cruise control in the non-adaptive mode for most highway driving. Even then, it is constantly playing the slow-down, catch-up game, especially when first engaging and on hills, which annoys me and I'm sure annoys anyone following me as well. I had a 1983 Chevy Caprice with better cruise control.
Lane-keep assist wanders aimlessly. I tried it a few times and gave up on LKAS as useless, so I turn it off and simply enjoy the car's silky smooth road manners. Lane-departure warning flashes at me and shakes the wheel at seemingly random times when I am well within my lane. Forward collision mitigation brakes sharply when passing behind a vehicle turning out of the lane ahead, even though the car ahead has already cleared the lane.
I want to separate things out in the discussion about LKAS and ACC since there are a number of different factors here. First I strongly suspect that not all LKAS systems have been correctly calibrated out of the factory because some comments just don't sound like the same system that I am using. Or even though we don't like to think about it cars can be damaged and repaired prior to delivery and we are never told about it, so I wonder if for example some windshields got broken and replaced but not calibrated.
I use LKAS all the time, and I mean it's on from the moment I start driving (even though it doesn't activate until 45 mph) and stays on until I turn off the car. I find it does a terrific job of helping keep the car centered in the lane. Experimenting with no other cars around it can even steer smoothly through a slight curve and come out straight on the other end. Others have echoed my experience. Not perfect and of course it is dependent somewhat on lane markings, rain, shadows etc. but it's much better than the Toyota systems that I have driven. The Gen 3 Prius for example had an LKA system that people loved (I never got to try it) but those who had it then switched to Gen 4 Prius (or the Prime) were hugely disappointed as Toyota ditched LKA and replaced it with the same LDW with steering assist system used in other Toyotas, which just crudely ping pongs between lanes. That was also my experience with it in rented Toyotas. So I have been quite happy with the Clarity LKAS. Can it be improved? Sure and I am encouraged by what Tesla is doing, and I certainly hope that Honda and others will be following their lead by the time I replace my Clarity. If not well then that would be another reason to get a Tesla.
As for ACC I think that one does become subjective and differences of opinion are based on expectations, I don't think it's because of miscalibrated systems (radar seems less susceptible.) I am happy to have any radar cruise control, it's something that I have wanted since first hearing about it over ten years ago (Prius had it as far back as 2009). My main goal was not having to constantly adjust the cruise when on the highway following behind someone who is not using cruise control, and it works great for that. I also find that it is a helpful safety feature in traffic as it helps me keep a safe distance between the car in front of me, so that's why I have it on all the time. Quite often it detects that the car in front is slowing down before I do. Which I like because even brief moments when you get too close to the car in front of you are vulnerable moments, and I appreciate that it helps me to always keep a safe distance. As with others I find that settings are key, for me I use Normal mode with ACC usually set at 3 bars (4 in the rain) and it keeps the distance smoothly most of the time. Not as smooth as I would like sometimes but still very useful.
Like any system (as far as I know) it cannot see everything ahead that I am able to see, like a traffic slowdown up ahead or a light up ahead that has just turned red. If the car in front of me is a typical driver they maintain speed until the last second then hit the brakes, and Clarity reacts rather abrubtly to that. So when I anticipate a slowdown that ACC doesn't know about yet, I simply hit the cancel button and start coasting or use the paddles or brakes. One huge peeve that I have is I would prefer to use the paddles to deactivate ACC but that triggers an annoying beep, and I just don't want to hear an annoying beep as a routine part of my drive so I use the cancel button instead which is not as convenient as the paddle, but at least it doesn't beep at you. So first I press cancel and then paddles, or just cancel if I just want to coast.
Acceleration is the weak point, it's often an annoying one or two seconds too slow to react and accelerate in traffic situations and I know it annoys the people behind me. Some people prefer how ACC works in Sport mode as it accelerates more aggressively, but I prefer Normal mode for other reasons. And like you mentioned when the car in front moves out of my lane and slows down, ACC slows down also, apparently not fully sure if the other car is clear of my lane. I anticipate this now and know to press the gas pedal in these situations. I can understand if someone feels like why do I have to do that, that's poor design. I can't disagree but I always feel like as long as it does a pretty good job slowing and stopping I can help out with accelerating as needed. I haven't really had a chance to try it on hills yet, if it works like you describe I may wind up finding it annoying to use ACC on hills.
