They should really sort the Materia issue better, I think.

Making them stackable is a good step, but since there can be many types of materia for each tier (I mean T1 being, i.e., INT+1, INT+2, INT+3, etc....), staking them doesn't really help much.

There are only 2 good solutions, imho:
1 - Make a separate tab for materia (like with crystals); [BEST OPTION]
2 - Make all materia of same tier have the same stats.