My personal experience so far is that the Lemmy community is significantly more responsive in getting rid of spam bots. I've definitely seen a few but they disappear in less than a day, and lately the amount of spam has been extremely minimal, almost nonexistent on my instance's all page. Compare to Reddit, where spam bots might get banned from a few subs immediately but would often take weeks to get sitebanned, if ever.

Most serious Lemmy instances require user approval to join via the short application, which means they have no bots at all. And the big key is that lemmy admins are quite active and talk amongst themselves; if an instance with open no-approval signups gets abused by bots, other admins will talk to that instance about it, and most of the time it seems to get fixed pretty quick. And if they don't.... well, you defed them until they do. It's pretty pog.

I have seen three rather obvious spam bots so far, all of which were almost instantly downvoted into oblivion and soon thereafter removed from the site. Granted, I mainly frequent niche videogame communities so I can't say anything about the spam/bot situation in bigger communities (like politcal stuff etc.) but for the part of the Fediverse I personally use, it is definitely an upgrade compared to the current situation of reddit.