You cannot change the abilities in the middle of combat, so if you are in grave danger and you didnt somehow knew before and prepared, you are screwed.
About tactics, Ive been reading all over the forum that if you do not have the skill mapped, the tactic will be ignored. (I have no confirmation on this though, although it makes sense)
I dont like this decision of mapping before combat, because you never know what you are going to face, and therefore you will always end up with the same 8 abilities mapped. This decision just kills situational skills, and gives nothing in return.
And I am starting to think this will also be true for poisons/bombs. We will craft a ton of bombs, but will only be able to map ONE. So we will just use the same one (the ones that does more damage) all the game. They could have removed the rest of bombs if that is the case.
I'll agree to the fact that it kills situational skills, if we have no prior warning. However, a lot of the abilities now seem to have additional properties which replace the additional skills, basically making lesser skills more-in-one. I for one like that, but that's me.
But I'm also thinking that for boss fights and dragons and the like, we'll get a lot of advance warnings.
Bombs, yes, but likely, everyone will have a favorite bomb and stick with that anyhow. Example, in the witcher 2 (yeah yeah, I know) you could also use bombs, and I loved them, more so than the signs, which I hardly ever used - other than quen.
But I still only used one bomb (dancing star) once I knew it also caused panic (oh, the laughs I got from people dancing in fire).
I know I'm kind of proving your point, but different companions will probably be able to equip different bombs, so that'll make a total of 4 different bombs and 4 different potions. So you'll still have plenty of variety imo.