BG was my main professional instrument for several years and then DB was with a fair amount of doubling thrown in. For many years now it's probably 95% DB. IMO DB is not really harder physically with good technique because you can use more of your body when playing it: core, big muscles, back, legs, all that. It's easy with BG to concentrate too much energy in just small muscles and combined with the tendency to bend wrists can be tiring and potentially damaging. I'm not saying every player does that but the danger is there. JMO
If you use proper technique on both instruments, BG requires less force and energy. Pulling from the core is still relevant for BG IMHO. There is no need to continually pinch the string between your fingers and thumb.
I agree that bent wrists are problematic with BG. Making a good choice on how high you wear the instrument can help with this, but IMHO not totally mitigate the problem. Raise the bass and it helps one wrist and makes it worse for the other. IMHO, all you can do is try to find the best compromise.
I don't see the wrist issue as related to the amount of strength required to play the instrument. In my experience, keeping the wrist bent for too long cuts off blood supply and/or impinges nerves, so you get tingling and numbness.
Unfortunately, I have done a few gigs where the band leader made us play three sets none-stop. About 90 minutes into it, I started having problems. My impression is the damage is cumulative, because after the marathon gig, it seemed like I would start to experience symptoms sooner.
Playing upright bass nonstop for too long is not necessarily a picnic. Playing non-stop for three sets would be brutal....I am not talking orchestral lit where you spend a fair amount of time sitting on your stool counting rests. I mean thumping the bass non-stop.
IMHO there can be issues with the wrists and elbows, but it's not the same problem and possibly causes less of a cumulative damage when you overdo it.