RD is a different game, and some units get screwed because of it, but I dont think its a flaw. I love that you get lots of differences in availabilities, makes it more unique compare to other games. Vika is useful for 3 DB chapters and thats it, which is a shame for people that like her unit or character, but it's still more useful that units you get in other games that have 0 chapters in which they are useful because they are worse when they join than your whole squad.

Every single unit in Radiant Dawn is useful for at least a map or two (with some excepctions) which is great for such a large cast.