Most private investigators make all their money gathering evidence on worker's comp fraudsters for big insurance companies. My dad's been a licensed PI for over 25 years now and domestic cases are few and far between (maybe 1-2 per year) because Mrs Smith pissed off about her cheating husband doesn't have the cash that Bigco Insurance does and PIs charge by the hour. The insurance company has the cost of hiring a PI for 3-4 days so they can win fraud suits with the video evidence built into their cost structure. Unless the payout from a prenup is really big, it's not going to be worth hiring a PI to follow your husband around after work for a week.
(Sidenote: If you're ever dumb enough to try to commit worker's comp fraud, you'd better keep up the act everywhere. Don't limp pathetically to and from your car at the doctor's office, then get home and work on your car or jump on the trampoline you have in your backyard. True story.)
As to the sidenote, it must suck for the people who have periodic pain bad enough for them to be unable to hold a manual labor job but who can still have bouts of living a normal life. Either they have to forgo the moments of freedom they receive or risk losing any disability.
Eh, it's not longterm disability cases that the insurance companies hire PIs on. Hiring a PI is expensive. The insurance companies don't do it until they are already positive the person is defrauding them. The percentage of cases my dad's worked where the claimant was not very obviously faking is vanishingly small. Like less than 0.5%.
By the time the PI is involved, the insurance company is already ready to sue, they just want extra evidence to solidify their case. They're nearly all acute injury cases - "I slipped and fell at work, and now my back's out and I can't work for six months" and they hobble their way into the doctor's office leaning on a cane when less than an hour ago they were doing heavy yard work (or jumping on said trampoline!) with no difficulty.
Believe me, I have sympathy for people with chronic pain/invisible illnesses. I have fibromyalgia, my mom has myasthenia gravis. We are not the kind of people that wind up with PIs following them around with a camera.
(Sidenote: If you're ever dumb enough to try to commit worker's comp fraud, you'd better keep up the act everywhere. Don't limp pathetically to and from your car at the doctor's office, then get home and work on your car or jump on the trampoline you have in your backyard. True story.)