Manchin I understand given he's from a deep Trump state. The goal of his job is to represent his people, who are gonna be skeptical of a lot of Dems agenda and frankly if he was voting just like any other liberal Dem taht would be concerning for the state of our democracy.
Sinema on the other hand represents swingy AZ, and several things she has held out from doing are widely popular in the state. Sinema's political journey is fascinating given she used to be very liberal and Green.
My main issue is not so much that she's "Independent", but rather she doesn't offer alternatives she'd be willing to go with to solve these issues that she seems to acknowledge are serious issues. If you're not willing to override the filibuster, come up with something you can get some Republicans onboard with, possibly like nationwide abortion exceptions for rape and incest or universal voter ID.
Jon Tester doesn't act like this despite similar circumstances.