... But it would suffer special problems:
The landing force could not Retreat if unsuccessful (Rule 7.6).
Good point. I had not considered that. You need a port in order to retreat/regroup.
This does bring up the question as to what happens to an invasion if no army unit lands in the first place? I suppose since the unlanded army units are considered 'naval' at this point then they could retreat navally back to a port.
If there was a Sea Attack on Naples through the Ionian Sea, and the defender had a Naval unit in Naples, the landing force could not land until it destroyed the defending Naval unit in Naples. If no Sea invader is attacking that unit, the invasion is doomed.
Of course the naval unit could be destroyed by air in which case the invasion would occur. But I agree that the rules are clear -- but weird -- in this case.
Rule 7.5 ... Invading army units are "naval targets" until they land, but do not count for naval stacking. They may land immediately after enemy naval units are cleared ...
FAQ 7.2 ... Naval units can only fight each other if defending/attacking the same coast.
So how does the defending navy unit actually fight the invasion force? The rules are clear that the navy defends but unclear as to how they would actually fight. The mind reels.
In any case my main 'use case' are the areas which are likely to not have naval defenders -- Spanish Morocco, Morocco, etc. Are they invadable from the non-port side? As John AnimalMother points out there is no rule prohibiting this thus it becomes another tool in our kit.