One of the first thoughts I had when making my game is I want an open world. Than I thought that could be a pain in the behind to track down bugs and other issues. That is why I have stuck with creating a bunch of dungeons. While that presents its own issues at least for that part of the game I can track down issues to an individual zone.
As far as animating humanoids goes I know Mixamo works for standard UE4 skeleton models and Unity should be similar from my understanding.
I’m not sure if Unity provides you guys with a “Control Rig” for whatever your standard mannequin model is. I have not played around yet with the one in UE4 that is supposed to help the ease of animation rigging. I suspect it is something I’ll be trying out this year.
As far as the errors, yes, those will drive you crazy they have me. If you get lighting builds errors for instance where it can’t finish it’s more than likely one of the free assets you are using. I myself have nuked an entire zone just to find the darn object giving me the issue.
Other types of errors are much harder to track down and deal with. I'm hoping if I can grow the game development community it will end up being a place people can try to get some help. So many errors out there have endless questions on development forms with zero answers. Leaving you stuck trying work it out on your own.
I have learned I want to keep my main project as clean as possible. If I’m trying to learn something new I tend to do it in a separate project. It costs more time and you can still run into unexpected issue conflicting with what you have already done moving it over. It however makes it easier when things don’t work out to deal with it.
Have a great week.