Back in Jan 2022, I got a Venture X card and did some testing:

In my test booking, I put in my FF# and it showed up on UA. I saw no issues with seating.

When I clicked cancel, it would not allow me to do it saying I had to go via TA.

When I did Change, it would allow me to go to the search with a warning that anything with a lower value would not qualify for a FFC. Once the change was completed, UA owned it. So it was pretty much like a FFC perhaps

This is slightly disappointing because I get a 1% "break from business" discount booking thru UA. The "change search" does not give this discount even on a UA ticket. I thought I could just book anything of equal price via the portal and cancel the flight for FFC and use it as payment booking on UA to get my discount and my portal rewards. I guess one must choose one or the other. It devalues the rewards slightly as well as this was the primary way I had planned to spend my rewards. I think I was able to do this using the Chase portal in the past but haven't tried it for a while. Now I mainly use it when I need to book non-UA flights like intermediate intl flights after flying in on UA. Even this is problematic because some low cost carriers charge you less if you buy ancillaries during booking vs after. Even hotels, they don't tell you if breakfast is included and it often costs more.

As for PP, the main issue is that the portal is extremely limited and you don't have a way to control the booking class. If you are trying to go for a W+ fare or even OAR, you might run into issues.

The C1 price protection is limited in dollar amount drop (I think $50) and the amount of days (10 days?). You use to get the money back on a price drop, now it is credit that must be used in the portal. When I first tried it, it would not catch drops. I would have to call in multiple times. It's basically only if their system happens to catch the drop. Maybe it's gotten better now.

Why is it hard to reprice on UA? You can use Change or just cancel and rebook. As long as UA hasnt tricked you into booking a non-changable to japan, you can reprice anytime for any amount.