It's certainly more difficult fighting on multiple fronts and against multiple enemies with Shattered Retreat than previously, but then that's the sort of situation that's meant to be difficult.
If you're big enough, you can deal with it (and you do get an advantage if you're defending - your enemies literally can't win without besieging you, and your armies will be reinforcing as they march about, because they're in friendly territory)
The system does respond well to tactical & strategic use of your armies (you need to focus your assaults according to a plan that will win the war, rather than constantly reacting to your enemy, and positioning of your forces is even more important given that counter-attacks are likely); wars aren't won with the first big battle any more, but that feels like a good thing to me.
Yeah, that's a fair criticism. I think it was probably worthwhile reducing the size of opinion bonuses (because vassals were a little to easy to turn from 'loathe' to 'love' previously), but some of the logic does need tweaking, and some of the bonuses / maluses are disproportionate as a result of changes to the rest.
I also think there's more that could be done with the favours system (there have already been plenty of good ideas for this on the forum), which should run in tandem with the opinion system rather than feeling somewhat separate.