I did a reclaim once, back in .44; I've also been to quite a few ruined forts in Adventure Mode:
1) yeah, there's a lot of clothes, but it seems (in my experience) that they're usually all "large" (sized for humans, I think) - so they're extra worthless, except maybe as trade goods. OTOH there's often a big pile of assorted goods near the surface (which may include armor + weapons), and usually some quality goods (including steel gear) in the middle section of the fort, in a room full of display cases.
2) It used to be forts mainly got ruined by being destroyed by forgotten beasts, meaning there'd be an angry "current resident" hiding somewhere in the fort (usually in a bedroom). So you'd have to go in prepared to deal with the critter. Nowadays I think most forts get ruined by necromancers, so there may be less danger going in (unless some zeds have stuck around).
3):
In my reclaim fort the lowest floor of the fortress bisected an adamantine spire, with the result that the lower floors of the fort were full of demons. We managed to seal them away, but it was a narrow escape.
Needless to say that's not
usually going to be a problem, but it's worth noting: be careful as you explore lower in the fortress!
Overall, I think the biggest pros are:
you get some free stuff, either for trade or use
you don't have to spend time digging out rooms
you'll never want for anvils
you get to interact with a place that has a history - nothing like reclaiming the old Dwarven capital for the full Moria experience!
The cons:
the generated fort layouts are pretty random and maze-like, so making a functioning, attractive fort out of it may be tricky