What did Vikings expect to realistically gain from Blót sacrifices aside from vague things like "good luck', "good harvests" or blessings in combat0? Was this sometimes seen as a waste of livestock by Norse people who may not have been as devout as their peers, thus sometimes getting boycotted?

I've been linking to this answer by /u/Steelcan909 a lot recently. The short answer is we don't know. We have absolutely no contemporary sources from early medieval Scandinavian pagan practitioners or believers discussing their rites, beliefs, dogmas, theology or strength of belief. All of our literary sources are either written centuries after conversion through a Christian lens, or by an outside perspective, usually one with an axe to grind.