A common problem with the late-80's FROs were cracks developing at the seattube pinch area and the chainstay loop near the BB junction. Yeti developed "fixes" for these that involved a gusset on the chainstay and a sleeve on the seatstay. These became standard on the frame design in 1990 (gusset) and 1992 (sleeve). Often times people would send their California purchased frames back to Durango for paint or repair and they would add these "fixes" to the old frames to prevent cracks from forming.
The one for sale has neither of these fixes, but it does have a front derailleur stop. When the frame was made, there were no top-pull front derailleurs available, and they had yet to add the pulley wheel, so the front derailleur was anchored to the chainstay and pushed with the housing stop on the lever arm. (as the one for sale is currently set up). The derailleur stop must have been added at a later time to allow use of then-available top-pull front derailleurs. As the headbage is about 3 years too new for the frame, one could assume the mods were made in Durango when the bike was repainted and the headbadge changed.
I don't see too many "problems" here. Besides the paint, the bike is a very solid old Yeti with many details collectors seek out.