Fatigue, fatigue, fatigue. This is what makes FE5 so challenging. Rather than just field your best units every chapter (read as Catria in FE12 from Chapter 2 onwards), it makes you rotate your units constantly. Why not do that?
EDIT: Actually, half of the stuff present in FE5 should probably make a return. Like PCC or Movement stars on shittier units to give me reasons to actually use them (Ronan, I'm looking at you).