If anything, the external ones are much easier to pull off and repair. On the other hand, they are a bit exposed to the elements but not that big of a deal.
I think one of the reasons for the external cylinders is that gives the manufacturers a lot more room under the seat, specially with cabbed tractors. They can use a smaller valve mounted on the side with the position feedback linkage easily attached to it and to the rocker arm, instead of the bigger rockshaft and cylinder assembly.