they weren't problematic per say but those folks that did have issues posted about them often. mainly because it costs money to repair them.
the early units had some issues with the clutch pack snap ring falling out/breaking/etc and that required a tractor split to fix $$$ and again people never post or talk about how reliable their equipment is when it's making them money, rather mostly when their equipment is broken down and costing them money
all of the grand L gst's were built mostly teh same in regard to the gst and all of the early ones were known for issues. Also, one MUST use super udt2 in them and nothing else. I don' t know what additive package kubota uses but if anything else was used in them, the clutch pack would chatter among other things, and the junk fluid impregnates the clutch pack friction plate material with the junk fluid and it seemed to be more frequently repaired, than tractors that used super udt. it wasn't just clutch packs either, it was also 3ph valves, loader valves, cylinders and a few other things. You could always tell when someone cared for their equipment and the cheap tractor fluid was one way to know that they really didn't.
with all that said remember that L35's are getting very old now and if you purchased, you WILL at some point have to repair something on it. The question is, are the parts still available, and do you possess the mechanical knowledge to do it yourself?