Adding 20 cards to your deck seems like a massive drawback that would hamper the consistency of your deck yet Yorion got the ban hammer in several formats, and some Modern decks still tinker with 80 cards, even without the extra creature in their sideboard
Whenever one or more Dragons you control deal combat damage to a player or battle, look at the top three cards of your library
Don't think it isn't a good fit for more experienced players too, though
The good news here is that both big and small red decks can take advantage of Purphoros, so if either one makes a comeback this God will see a large uptick in popularity
Modern Horizons will see more high level play than any Masters set has seen, which warrants a deep look into this new, high-powered format
Perhaps it traveled to Kaladesh, taking on this new full form, in search of something and has yet to find it