It is mostly like it was at the beginning of the Legion (I'm looking at you Cenarius). You spend all of your runes just to maintain bone shield up, use blood boil and hope for proc for DnD. Heart Strike is usable in ST time to time.
As for "I can't do anything for 5 sec" - it happened to me playing with blooddrinker as well. Changing to Rune Strike quite helps. If I don't spend all runes at once and plan a bit ahead, I have to wait at most 2 - 3 sec to be able to cast anything. I'm not saying that the "anything to cast" is always what I really want / need to cast
Anyway as fresh 120 lvl BDK my feelings are:
solo content is slow, as dmg output is very limited with maintaining bone shield, which drops really fast with 3+ melee mobs. With 5+ mobs I had some hard times to even survive. Magic damage and DoTs hit like truck at 4+ mobs and I have use defensive CD just to kill few mobs. This is maybe my fault, as I comparing this with live
In dungeon (just normal, don't have gear for HC yet) it seems a bit better, at least it feels better. Once you don't need to take care about dmg, but purely for survive, it isn't that bad.
Imba healer with BDK is disco priest, never have any issue with him in group. Paladin seems a bit undertuned, or maybe I had bad luck on player