And even if you do understand the pros and cons, it can be complicated because all these instances have differences and you have to figure out which one has the things you want and potentially make compromises.
Eg, I knew I absolutely must have downvotes and several instances disable those. I wanted to be federated with both lemmy.world and beehaw (and also lower my risk that beehaw is gonna defederate my instance in the future). I wanted as large of an instance as possible because by “fun” design, the “all” feed gets better the bigger the instance (IMO a design flaw), as does the ease of subscribing to communities (being first to subscribe is harder).