TBH, the Slim is also very reliable overall, but it is
a lot easier for dirt and whatever else to get into the disc tray and drive. I've also heard that it's possible for the mechanism that controls/holds the Slims disc door to scratch discs if it gets damaged, but I never personally experienced that with either of the Slim PS2s I've owned.
That being said, I've always gravitated towards the Fat PS2s, because I had one for most of my childhood. I also love all the funky color variations that the Fat models got. At some point I'd love to have an Astral Blue PS2, but it's currently a little hard for me to justify dropping ~$1000 on something like that.
I own a PS2 Slim, got it like 20 years ago as a birthday gift.
Never had any major issues with it, but from my personal experience I can tell you're very much right about dirt getting into the disc tray (and even on top of the laser lens).
However, my Slim isn't exactly in its original state... it's modded to run "non-official" CDs and DVDs, which was a very common thing to do back then.
That means I can play a game with my disc tray completely open, with some inventive use of duct tape, a coin and a piece of paper... 😅
Also, there's another issue I've spotted on my Slim: there is an orange strip inside the disc tray, I think it's connected to the laser unit, and as the laser goes back and forward to read the disc, this orange strip can easily start to go up and scratch my disc as I play.
I noticed this only after hearing this very strange "scratching" sound while playing... and sure enough, some of my most beloved games are already showing quite a few scratches on the surface.
Can't seem to find a solution for this. The only thing that seems to work is : use the DVD I've been given to bypass the system and allow it to read non-official DVDs, use a piece of paper on a very specific place to trick the PS2 into thinking the disc tray is closed (but it isn't), run the DVD so that the laser unit comes foward, remove the DVD, push the orange strip down with my finger, and then put in the disc I actually want to play... switch off the console and on again.
Might sound tricky , but doing this seems to be enough to prevent that orange strip from scratching my games... thank God my PS2 is modded, otherwise I couldn't do this work-around.