We've got one with around 5000 hours on it and I can't think of anything unusual to watch for.  Like any articulated tractor you need to watch the pivot pins.  If they were not properly maintained (ie greased daily) they can wear.

The biggest problem we have had to date was a Cummins problem.  The pistons were made in 2 pieces and then glued together.  The problem is after a period of time the pieces "de-bonded".  Caused a bit of a problem.  By now any engines with the problem have probably blown up.

Replaced the water pump once ...  not a big deal.  Replaced the HVAC coils. replaced the cab roof liner when it fell on my head.  Really nothing too unusual.

Tomorrow might be different.  Right now it is setting waiting on a truck because the hydraulics have failed.  Mechanic pulled the pumps off on the field to find a splined shaft with no teeth.  Most likely will require transmission removal to repair.

A free tip to remember if you buy it.  When unhooking hydraulic hoses from the  quick couplers don't try pulling them out.  You'll tug and strain and discover new bad words.  Push forward like you are putting them in the coupler, hold the hose on the back so it can't move and the coupler almost falls out.