The truth is both Silver and Blaze don't really work very well as Sonic characters and have very niche and limited uses. Their powers aren't Sonic-ish like the other characters, being just generic superpowers that don't have many interesting uses gameplay wise. And their roles and settings are very limiting. Silver in particular is a very uninspired character that doesn't work outside the context of his debut game. Blaze I feel can be used sparingly since she's more interesting. Ultimately they're both attempts to recreate Shadow in some capacity and both fall short.