Well, in my game (VH/VH) both Scotish and English attacked me by see. They DO use boats and landings and where very strong. I dont know how they did it, but they dint fight one onether.
Any way i thing that HRE has one of the best units in the game. A couple of imperial imperial, mounted and dismounted, some armored sergeants and couple of passive x- bows and you can handle just about everything. The only trick here is that you need more spears against the Mongols and the handle, and that is it. Afcourse your city army's suck, but not that much. If you dont like it check the Russians, the Pols, and the Hungarians.
O, and HRE has the best economy in the game. Banks and printing presses. You have a ton of cash and one of the best armis.
So yes, you really can play slowly whith them, but only after you take all the rebels, France, Britain and Northen Italy. After that you can wait until the end of time getting richer and snobby.