It is intentional that the armies can do the coastal moves as well as the fleets. Small scale ferrying actions, particularly in the west were common to the era, and are also necessary for map balance.
One thing that I am not seeing much of (that I expected to) is players supporting neutral armies to hold as buffers. This is a critical element of containment in the design, both as a strategy to deter Oda from heading towards a specific peripheral power, and for Oda to defend against a strong alliance of either the northern or southern pair. This is both historical (the spiderweb of minor daimyo alliances) and for balance. The peripheral powers need to view and use at least some of the neutral armies as their own for containment, even if that delays their own growth slightly. The starting unit count of three for peripheral powers is intentional with this in mind.
It is also definitely intended by design that if Shimazu and Mori fight each other, and Uesugi and Hojo do the same, that Oda definitely should steamroll to a quick victory. Likewise, there is extremely high border tension built into those peripheral pairs (exacerbated even more so by the coastal rules) so that they assume high risk when they do work together.
All that to say, reducing Oda to 5 is definitely on the table if the playtests demonstrate a need, but a significant number of the games I'm spectating right now suggest that it may not be necessary, either in gunboat or full press. The two that have been finished so far certainly had positive outcomes for Oda, but that is not the case elsewhere. More to follow on that when other games finish, we can't discuss the specifics of those yet.