Compared to a direct connected locomotive, a geared locomotive is a piece of cake. For one thing the cylinders, cranks, and eccentrics are ridgedly attached to each other and to the frame. The only way they get out of "tram" is if the bearing suraces become badly worn. Also, the cranks are not weight bearing members. There are no shoes and wedges to get out of square, and the rods are very short and therefore much lighter. The down side is that you have to crawl under the locomotive to work on a Heisler or Climax engine. On a Shay everything is on the right side.
Working on the runnung gear is equally simple. You remove the foot boards and center pin keeper, disconnect a U-joint, jack up one end and roll out the truck. No need for a pit. In fact this was often done right out in the woods. We have accomplished this many times on our Heisler useing wood blocks and bottle jacks. Once this is done it is just a matter of removing parts until you get to the offending ones. Again the relatively light weight of the parts allows this to be done with a chain hoist or forklift.
I don't mean to say all of this is easy, but it is simpler than dealing with a rod locomotive. Geared locomotives were intended to be maintained by less skilled personel, using relatively crude facilities.