I bought one and installed it like the OP did, used this page as a guide too. The way it works vs the factory 3 point I trust it enough to hold me in place and do its job.

That said I have the factory harness also for street use because.. well its more comfy.

Ive had this setup in my FRS for about about 4yrs now. Love it. I did a ton of research before buying it, and believe in the ASM tech enough to run it. Mine is mounted exactly the same way, and its terrific.

I don't use it on the street because why TF would anybody really want harnesses while cruising in traffic?

But this mounts so conveniently to stow away when not in use, and it takes like 45 seconds to get the OEM seatbelt out of the way to set this up for autoX or track.

Love it.

Is this because of concern related to rollovers or submarining.

Rollover in particular, but also exit speed. ASM works, but I'd be wary of it on the track just because it can be so bad. But I also don't feel comfortable without a Hans on the track. Honestly surprised we didn't have bad issues from the autocross incident.