It’s terrifying because nobody knows the limits of its use and therefore nobody can say with any certainty that it’s both (a) technically safe and (b) won’t disturb the network ecosystem. 

My understanding is that taproot and segwit’s witness discount encouraged and increased the volume of arbitrary data stored on chain. 

This expanded the market for things that use this data. More data, more junk, larger market. 

Thereafter private apis to miners and pools developed to front-run and insert crap in the chain. 

Private apis and out-of-band payments encourage mining centralization by giving unequal access to fees and disturbing the even playing field. This motivates miners to join the pool that has better access to these exclusive fees and encourages monopoly-like arrangements. 

https://www.coindesk.com/opinion/2024/07/09/mev-has-spread-to-bitcoin-in-subtler-forms-than-on-ethereum/
 
https://bitcoinmagazine.com/technical/the-witness-discount-why-some-bytes-are-cheaper-than-others