I work at a large referral and emergency hospital for the 3 board certified surgeons. If they can, they prefer to do a THR over an FHO with a larger dog (and honestly smaller ones too, since the smallest we've done is a boston terrier) because they have a true joint instead of a false one made of scar tissue and shifts within the muscles to compensate for the lack of the ball and socket style joint. You have to do passive range of motion with them because if you don't help them get the leg moving (which honestly most FHOs that I have seen don't want to put weight onto that leg even a few days after the procedure) the muscles will not end up in a functioning manner, but rather just tighten and stiffen up. It's the big issue with femur fractures also, the muscles can get extremely tight and then its truly hard to get them working right again.
It is, as you say, a salvage procedure. Which generally means last choice. If the OP feels comfortable with the veterinarian they are working with and with getting the THR done, then why shouldn't they? I never regretted getting Kenai's hip replaced. I saw a dog who had to bunny hop to run, had her legs literally fall out from under her, run like she was a normal dog once she had healed from surgery. It was like she had a third leg to stand on, and I still feel immense joy watching videos and seeing pictures of her running after the procedure.
Both procedures still have to heal. You cut bone and shifted muscles. The body has to learn how to adapt to a new configuration. The loss of the joint that it relied upon. You still have healing, the patients still experience pain. We wouldn't be running them on fentanyl CRIs or sending them home with things like fentanyl patches, tramadol and morphine if it didn't hurt! Not to mention NSAIDs like rimadyl and meloxicam. Orthopedic patients generally go home with a good two weeks worth of pain medication and anti-inflammatories.
Veterinary medicine has come a long way, and the advances that have happened really are for the better. Our patients with THR can walk, stand, etc on their own. We exercise the caution to give the bone the chance it needs to heal. Specific exercises at different times of recovery, acupuncture, lazer, e-stim, hydrotherapy. Any surgery that deals with bones is only as good as the after care you put into it. -shrugs- I may be a minority voice, but I do believe that THRs are not the evil devil everyone wants to make them out to be. And my reasoning is from helping with the procedure and after care many times working with the exceptional team of vets and techs that I work with.