I've had a ton of 5 series saws, right back to the beginning running the 576.
Very few problems with them, outside of what you'd have with any other saws.
In the early 550 and 562 there were certainly problems with the fuel heating up, but by 2014 or so they had most of that worked out.
The biggest issue I see is people missing the pop when starting them and then they flood them out. I have a buddy who send me a text now and then so I can walk him thru the steps of how to start his flooded 545mk2, it's been a while, should be getting one soon lol.
My favorite 50cc saw for limbing is a late model 550mk1, I have a couple. I also have a 550mk2, 2260, ported 562, 572(could be missing something), I like them all. In the past I had a few different 540xp, they were good saws too.
Never owned a 585 or 592, but the guys seem to have muffler problems with the 592's.