Different approaches have different outcomes.

With Liquid, the design has members of the Liquid Federation Functionaries to take turns proposing and signing blocks. You don't want other Functionaries to refuse to confirm the block because they don't want to earn the fees in Asset X, and Liquid's Federation model would be harmed if proposed (valid) blocks aren't getting confirmations.

What are the different types of participants in the Liquid Network? https://help.blockstream.com/hc/en-us/articles/900002163803-What-are-the-different-types-of-participants-in-the-Liquid-Network-

Sequentia's design is, from my understanding (after a cursory read), accommodating to where there is a market for block candidates and the validators will agree on a choice of one from that market to use. If validators don't want to be paid in Asset X, then they would not agree to a block that has transactions using Asset X for the fee.

I could see how that gets abused though. What if Asset X founder bribes validators to choose blocks where Asset X is paying fees giving those blocks an advantage over other blocks. I didn't read enough on it to know how (or if) that is addressed, or maybe why that isn't a concern.