Yes, but you can also reclaim your space. I've configured nix to only store two latest revisions and have a ./switch.sh file that automatically clears everything up after applying nix.
I had issues when giving boot partition only 512mb, so I'd recommend going with 1G.
Yep disk space and learning curve are the two major downsides to Nix. The former has never been a problem for me in practice, just run garbage collection once a month. The latter was a big problem, but is now mitigated for most people by LLMs.
Yes, however the space is not „used up” in a classic sense. It’s a cache, so you can give up some of it and reclaim your space. Fresh after a full cleanup it won’t take much more than a regular distro.