Rielle decks often focus on card draw payoffs and making the commander into a massive threatit doesnt take much for Rielle to threaten large amounts of damage, especially with access to tricks like Violent Urge and Temur Battle Rage
Even if we were doing Winding Constrictor shenanigans, this is just too slow and ineffectual
Of course, good will alone wont win you the game, so Font of Mythos will only be a good fit in decks that know what to do with the extra cards they draw (or how to punish foes for being flooded with cards)
Slinza also gives the beasts entering a +1/+1 counter, attempting to bring some older Beast creatures up to par and engage with +1/+1 counter shenanigans
At the beginning of your end step, if eight or more cards are in your graveyard, transform this enchantment
Seedborn Muse and Drumbellower make this Naya commander sing with value