I recently played a game where boths sides had only RMUs and SMUs...about a dozen models a side. It worked brilliantly!
That looks a great game!
The point I was trying to make is that if you get to a stage when it's
only SMUs, the unit-based mechanisms could start to feel a bit inflexible compared to SoBH or other "single-figure" systems. I don't think RMUs matter at all in this - whether a unit is two (like your shield maidens) or four or six or twelve isn't really significant.
I mean, DR is a great ruleset and any game played with it will still be fun. I'm thinking, though,, of the various treasure-hunting scenarios, where the lack of freedom to have one character sprint further than usual or climb and jump or shoot then dash or duck might prove a bit frustrating. But as soon as you have some groups rather than all individuals, though, the objection entirely disappears.
The ability to make small units out of just a few figures is a terrific facet of the game - as is the reverse approach (oversized units). A "heroes against hordes" scenario with undersized units versus oversized ones could look brilliant on the tabletop.