Well not the only problem, there's also the fact ships are limited to water deep enough to carry them, so you can't go too far from the coast or navigable rivers. Granted, in Europe, there are lots of navigable rivers that make waterborne logistics fairly simple if you have the boats. Ask the Vikings.
This is mitigated by the fact that rivers are attractive places to live, so the odds that a target of conquest is on a navigable river are fairly high.
Gets pretty bleak if this isn't true, and if your foe doesn't have to stay put, much worse. The Chinese tried to scour the horse barbarians of the hinterlands a few times. Didn't go well.
Well not the only problem, there's also the fact ships are limited to water deep enough to carry them, so you can't go too far from the coast or navigable rivers. Granted, in Europe, there are lots of navigable rivers that make waterborne logistics fairly simple if you have the boats. Ask the Vikings.