A single card that generates mana and draws cards makes for a pretty powerful all-in-one spell
Those same decks also end up having a bunch of permanents, so Emrakul doesnt always wipe their board clean
There are still plenty of ways for the deck to pop off, however, which could even earn it a spot at cEDH tables
If the item does not exceed several tens of thousands of yen, they either blacklist all bidders and end the auction, or have it won by an account they are using for self-bidding at 1 yen (likely to avoid fees)
If the enchantment was an Aura, you get to untap Yenna and scry 2 but you can only activate this ability at sorcery speed, making it a little lower power
Cycling for 1 mana is much easier to squeeze in, especially on turn 1 if you have nothing else to do with your mana