I was thinking about what action combat has that tab doesn't and the main 3 things I could come up with is verticality, precision aiming and action dodges. Are there any tab games that utilize those mechanics? Can you think up any more features that can only work in action combat? Cause I think I can come up with a few intricate boss encounters that use all three of those across all types of classes, though I'm not sure if I'd be able to make hundreds of unique fights with them. But then again, outside of now infamously awesome and incredible EQ(2), I don't think there's any pve mmo that has over a 100 absolutely unique top lvl fights.
This is probably not the most useful way to look at this anyway. I'll just 'complain' again. Even discussing 'Tab' vs 'Action' is meaningless. It's like getting two buckets for collecting sand on a beach and then labeling them differently based on which TYPE of random seashell they contain when you scoop a big pile of sand up.
Just ignore all of that. If you want good bosses, design the experience you want/need the players to have and work backwards, and whatever comes out, build. Ashes' encounter team is doing this already. The only question here is 'whose expectations are they going to annoy?'
'Action' Players - "I dodged that but the AoE is so big in this game that it doesn't let me truly dodge!"
'Tab' Players - "I can't do anything because I have to be moving around all the time like a maniac!"
Or they could just design it properly (evading to the outer range of abilities lowers damage for example) and make BOTH those extremist sides upset like they already seem to be doing. Same thing for higher end PvE.
"16 man content? That's barely a raid at all! Where are my mechanics?"
"40 man content? This is just DPS check vs random wipes/convoluted mechanics/add swarms."
Somewhere in there is a sweet spot where you just have to accept that a tradeoff had to be made and live with losing whoever is hardcore against it. And if you say 'Hybrid' Combat, low mobility, Split Body animation, you've basically already decided what that spot is because you design these things by designing your goals and constraints, not 'I want this game to have good PvP' or 'I want this game to have good PvE'.
That'd be the most realistic reason why it's taking so long, too, wouldn't it? Combat systems don't take THIS long. Too many good bases that you could just reasonably 'copy' if you knew your targeted constraints. They're just probably working out those things still.