Yes, I think all PS2s have BC. This was back in the days when Sony was saying that they wanted "to keep the format going forever" with backwards compatibility, back before they hit a brick wall with the PS3's design troubles and density scaling / cost...
Three things come to mind.
The first is that some PS2 slimlines were recalled due to a power transformer that can overheat. Unfortunately I don't have more detail to hand but it is mentioned in Wikipedia:
https://en.wikipedia.org/wiki/PlayStati ... se_designs
Second, I recall Fudoh mentioning that some slimline PS2s may show more noise in the RGB output. I didn't notice this when hooked up to a XRGB2, so your mileage may vary.
Finally, the big issue with all slims is probably down to the laser module dying, on which I have only seen anecdotal evidence, but repair folks definitely have their favorites here. (I don't know which, off the top of my head.)
I like all my slims, but I would give the nod to the 770XX or 790XX as I am a fan of separate power bricks. But I've got a Japanese 900XX and it works fine too.
TBH, I like to hoard all the PS2s I can find because they aren't getting any easier to find.