221 clearly under performs for cost no matter how much you want to put on it.
I suppose the difference is you're looking at the 221 and 223 as two separate units.
I see them like Obersoldaten: they in effect cost 340 MP 90ish MU, but you only have to pay the manpower to build them.
The Obersoldaten unit without its MG34 is hardly worth 340 MP, let alone the old 400, but it's still useful. Once you've paid the rest of the cost for its MG34, it leaps up to full effectiveness. It's the same with Rangers: naked Rangers just aren't anything to write home about.
You don't build a 221 to keep as a 221: you build it to provide some helpful fire support before it turns into a 223.
You could easily have the 223 as a 300 MP 30 FU unit you can only build after upgrading an HQ, and it would end the 221 complaining. But it'd be an effective nerf to the unit.