The problem with Anubis is his huge cost. If I could make him the last card in my deck every game he'd be amazing, but if you draw him in the first 20 cards he's either going to be discarded or just sit in your hand, not helping you survive.
That said, if it makes the grind more interesting for you then do it - having a random card or two in rainbow won't throw the balance off so far that you won't win, it'll just (generally) make it slightly worse. Keeping it interesting is gonna be more important than the odd win here and there when it comes to FG grinding.
This is mostly just opinion ofc, and adding him may result in people making variations of your rainbow in a months time instead of SG's. Experiment till you find the build that suits you