The biggest hurdle I've ran into is that GPS resolution just isn't accurate enough. If you want something that can follow a planned route without having to rely on a bunch of obstacle avoidance (which is difficult enough in itself) then you'll have to set up some sort of local RTK in conjunction with the GPS.
Currently I'm just setting my mower up like a FPV drone that I can drive around remotely from the comfort of air conditioning. Hoping someone comes out with a good automated solution.