What, besides coefficient of expansion, could make two pieces of glass incompatible for firing together?

No idea if anyone here will have this info for me, but I figured I’d try.

When firing glass in a kiln, it is important to use consistent coefficients of expansion within a piece. If one layer expands more quickly than another, the piece will break.

Are there any other factors that could make glass incompatible? Melting point, maybe?

Before today I only had Fuseworks brand glass, but today I got a lot of wonderful glass of the same COE, but a different brand. Now I keep reading that Fuseworks glass isn’t compatible with other glass, even of the same COE, and I can’t understand why. I’m wondering if this is just their marketing strategy to discourage you from buying from other vendors.

Maybe I’ll just have to try it and find out, but does anyone have a clue?