Yeah, I agree whole heartedly about the walker's death effect. It felt very Harry Potterish. I think, like you've guessed, it's just a case of picking where effects money goes with a limited budget. But in this particular case it comes off as jarring because the show is otherwise so graphic, and so this style of effect which normally gets used to ensure a PG rating feels out of place. The more decayed undead felt kind of LOTRish or Potterish as well. I think this same complaint was common after the episode last season in which Brann and Co. had a skirmish with reanimated skeletons.

As for the giant's animation being recycled, I think it was well edited, because I wouldn't have noticed if you hadn't pointed it out.