[MENTION=6804070]LordEntrails[/MENTION] - of course it's possible to come up with a convoluted set of circumstances to "explain" the presence of the Hill Giants, but that doesn't make it logical. It just makes it possible.
You keep using that word. I do not think it means what you think it means.
Yeah, I'm glad that works for you but it's too much of a reach for me to convincingly sell my players. And that's the bit that bugs me. I've got to roleplay these damned NPCs and if I don't buy their motivations (or I'm not given them) then how can I possibly do a decent job at the table - without doing even more prep than I already have to!. And that's where weird encounter setups start being annoying.
But, at that point, it's YOUR problem. Since other people can run it without issue, why would WotC cater to you? It's annoying to you. At some point, you have to accept that you are the source of your problems.
I've been DMing for a couple of years now so I don't have the experience many on this site do. While I've yet to run it I think OotA is the most thematically coherent adenture so far. I don't remember anything leaping out as wildly out of place. Running an adventure of course reveals issue that were missed during a read through. While CoS appeared good the shambling mound in the Death House was an early WTF?! for me and there were others too (like zombie houses littering Barovia). Caveat I've not run that one either.
Whereas our group thought those were FANTASTIC. Evil plant creatures in a haunted house trying to eat us? Cool. Shades of Evil Dead right there. Good grief, evil trees eating people is a staple of horror. It's all over the bloody place. Why on earth would you have any problem with this?
Zombie houses - or rather houses of evil - in a Hammer Horror inspired landscape? Again, straight out of genre. Good grief, it's almost cliche. How many evil houses are there in the horror genre? I take it you've never played Silent Hill or any of those genre games, huh?
Hrm, two stupid guards... yeah, that's totally immersion breaking. We'd never, ever see stupid people be put on guard duty would we? It's completely outside of genre... oh, wait, no it's not. It's again, darn near a cliche of the genre.
Look, at the end of the day, no module will EVER be 100% usable at the table without changes. It just can't be. Every single one will have to be tailored to your DMing style and your group's playstyle. That's just the nature of the beast. Always has been and always will be. If you want to run modules, and you don't want to feel frustrated, simply accept that you are going to have to do some work tailoring that module to your group. It's a fact of life.