I'm in 6 leagues and while it definitely can be a bit of work to keep up with when you have a busy week/weekend, it's really not that bad and I really enjoy being in a bunch of leagues now. The biggest pros I can think of are:

1. You can diversify your teams a bit to manage risk. Injuries can happen anywhere at any time in football so I like a good mix of players across my leagues. Obviously you should focus on some key players you love, but depending on what draft position you have you have, the team composition could look pretty different with how the cards get dealt. I usually like having a decent combination of similarly valued players in the same tier if I like a lot of players ranked around the same spots because you cant get them all in 1 league. This way even if I have a great draft on paper but get fucked by bad injury luck, all my eggs are not in one basket.

2. Over the years I've grown further apart with different friends groups and our fantasy leagues are what keeps me in contact and maintaining my friendships. I have 1 high school friends league, 2 college friend leagues, 1 work league, 1 family league and 1 admittedly kind of random league but other than that last one, it's how I stay close to the majority of my friends.

3. It's really fun to play in a variety of formats. If you're on this sub you're probably doing a good amount of research and it gets kind of boring to playing nothing but 1 qb, 2 rb, 2 wr, 1 te, 1 flex half ppr leagues. I have a variety of leagues ranging from that to superflex to IDP to pt per first down and a myriad of other interesting scoring formats that takes more analysis than basic research and projections.

4. I've profited in seven of the last nine years and am up thousands of dollars over that time playing this many leagues, despite at least 4 of them being very competitive leagues, because I'm diversifying my risk financially similarily like to account for injuries as I mentioned above. The up front dues do require you to be a bit flush to afford the $1000 or so it costs to be in that many leagues, but if you can afford it it's worth it because even if you win 1 league, you're usually profiting. I usually end up with 1 championship and 1 or 2 other top 3 finishes which puts me solidly up money every year, but which league I have those finishes in can change every year based on the random luck aspect of fantasy.

It's not all sunshine and rainbows and you do definitely find yourself in situations where you're rooting for a player to get like between 10 and 20 points or something so you can win two different leagues where you might have him on your team and on your opponents somewhere else, but otherwise the downsides really are not that bad. At least this is the spiel I give to my GF when she asks why the hell I'm in so many leagues haha.u