Well so far it works because they've extrapolated on what the characters can already do and "generic sword slashes" are easy to create, all the normals Katalina and Gran use aren't from the games but it's easy to believe they'd do that. Then their specials and supers are based on their in-game skills which works fine.
But Silva is a sniper. You can't have "generic sniping" as normals, that wouldn't really work. Then you could make her normals kick, since that's what her normal attacks are, but then that means 90% of her moveset is kicking, which isn't what we think of when thinking about Silva, it doesn't fit her image. Then on top of that her specials don't work in a fighting game.
Meanwhile I can easily see Zeta and Vaseraga being in by giving them generic "spear attacks" and "scythe attacks" for their normals and their skills convert well to fighting game specials. Or Cagliostro has enough different versions that you can make an entire moveset around all his spells and different basic attacks.