I think assimilation is too unprecise of a word.
In the context of these discussions nobody ever complains about foreign immigrants with a small army of kids running around and they're (at least initially) as un-assimilated as one can be. Heck, they might not even speak the language.
Meanwhile everyone agrees that Karen the corporate lawyer who shows up and promptly gets to work building a fence, calling in noise complaints and narcing on everyone for ordinance violations and unpermitted work is in the wrong.
I'm struggling to put my finger on exactly what it is but there's this disdain and unwillingness to respect local norms that makes the latter example of a newcomer toxic and unwanted (and IMO it seems to correlate strongly with wealth of the newcomer).
Agreed, but it goes both ways. "We welcome your money, but please keep your opinions to yourself", isn't a great way to assimilate new migrants. Also, see top comment by Mark Watson.