To be honest I don't think I have EVER seen Astarion lose his temper with me, in countless playthroughs both as Spawn and as Ascended. The most he ever gets is a touch grouchy if you're doing all the nice options and not talking to him and that's not in his conversations, its just remarks he makes as you're travelling sort of thing. Although I have never compared him to Cazador - why on earth would I do that, even Ascended he's nothing like Cazador. And I have never broken up with him - but I've seen it on U tube - he shouts, that's about it really, then goes into a sulk for a couple of days (why would he be expected to do anything else? You know many people who take a break up with someone they love really well and don't even go sulky for a day or two?) then he's back to normal.
The people who are against Ascended (and even more so the ones who have never even played that path) seem to make things up a lot. Maybe they hear what they expect to hear rather than what is actually said or they pick argumentative options because they expect the argument so go out of their way to get one - who knows?
