There are a number of reasons that a deck could be flagged as invalid, including not having enough cards to meet the minimum number required or too many cards over the allowable maximum
It amplifies damage dealt by your other sources while also dealing damage itself, it has lifelink, and it plays exceptionally well with burn-based spellslinger payoffs like Electrostatic Field and Guttersnipe
Dont be afraid to sacrifice quite a bit of your board to cast an early Vipereven if they do remove it, squirrels is a deck that can recover from behind quite well
The song released a year ago, and I've probably listened to it 3000 times or something"
Especially in multiplayer formats like Commander, having your full mana pool and complete array of blockers primed for your opponents' turns is so useful
Nonetheless, it is still very popular in Stun decks, in which it usually protects the backrow floodgates from enemy removals