But a diver *should* be prepared for the possibility. Physicians that I have talked to that have dealt with patients having diving issues say that how a body reacts to the physics of diving depends on your physiology and that yes, come people *should* do decompression stops because their chances of having issues due to illness or age are that potentially that high. One went so far as saying that older divers shouldn't fly within a day after diving, something I follow.
I'm 'older', and love diving (It's better than sex (sometimes)

) and want to keep doing it as long as I can, and also not end up paying for my love of scuba diving. So diving with a 'dive computer' could hold someones hand as they walk off a cliff. Dive safe...
Regarding the mechanics of measuring depth, it carries the similar issues with the ways of measuring altitude. It's a port, a hole, a small hole. Sand, grit, life, can block that hole. Soap buildup, etc? In planes, it's usually either insects or stupidity that cause them to fail. I could see an Apple Watch being gummed up with sunscreen, blocking the depth sensor port either fully or partially, and depending on how deeply recessed it is, fill the void. Once gummed up, what cleans it? Iso Alcohol? Acetone? Is there a way the Watch can warn the user that the port is likely plugged. People obviously should not be sticking things in that hole too. It has been discouraged to wear your dive equipment on a daily basis, so *if* someone is depending on the Ultra as a dive computer, they could be misinformed. It's a safety device at that point. *shrug* I would think of the Ultra as being an accessory, not a primary or sole safety device. It is good to have a backup/redundancy as you mentioned. I dove with 2 (sometimes 3) depth gauges for example, one the computer and one or more a mechanical.
The thing is, I never know how close I will get to the limits and try to take it all into account. I appreciate dive crews that take safety seriously. One dive in the Caribbean, they hang tanks at two levels, and a couple people bypassed the lower rack but they tried their best to make sure everyone stopped at the upper one. It was being over cautious perhaps, but for people that could have done several dive that week, and/or were flying soon, talking safety seriously can be a buzz kill, but also warranted. You might be surprised at how many people have issues after coming back from vacation that were related to their diving/flying. And some have had to be 'tanked' to recover. (There was a physician downstate that happened to work at a hospital with a chamber and ended up being a resident for a short time after personally not recognizing the symptoms upon returning from vaca and being hit with the 'surprise' diagnosis by a colleague)