I can't comment for everyone, for every GP or cardiologist/nephrologist who is informed by their patients what their smart ring or watch says about their BP or cardiac rhythm, or for all the devices offered that purport to take these various readings for their wearer. I can only comment on what I've read and as a Galaxy watch user now for 9 years:
The watches, worn as they are instructed to be worn, reasonably snug, with regularly cleaned back where the optic sensor lives, and placed just behind the big wrist bone, do a pretty solid job. They have all been approved, if they have the BP app installed, by a governing body that ensures they do a reasonable job.

They'll almost never completely agree with your doctor's sphygmomanometer or their auto-cuffs if they use them. Because of so many of the variations between patients and doctor's devices, we should all anticipate this and accept it as a fact. What they are intended to do is to provide the wearer with a running 'commentary' of trends, of spikes or standout readings, as they are revealed by repeated measurements over days, weeks, and months. If the trend is rising BP, that should tell you, and your GP when you tell her, that something untoward is going on. You can take 15 readings all day long, every day, and never have quite the same readings, even in the same chair doing the same things. It's natural variation, something your GP's office device is also subject to.

Again, speaking only about my own experience, now on my third Galaxy watch (they change each year, and I really appreciate the rotating bezel, something Samsung seems to do away with every other year. When a new model arrives that once again boasts of the top rotating bezel, I buy one and stash it for future use...call me crazy...the Galaxy must be recalibrated with your own home cuff every 28 days. The app gives you a reminder for three days running, and then disables the app until you recalibrate. I don't know if this is empirically driven, I suspect so, but that's my experience; the watch must be recalibrated about once a month.

My electrophysiologist accepted, without question, and without asking me for a formal ECG at my hospital or cardiologist's office, my contention that my watch had found me in AF. I am symptomatic and know when my heart begins to do the 'floppy chicken'. I felt it one evening, took the ECG per the watch's app, and then called his nurse who would always take my calls. She asked me to upload my watch ECG and send it via an attachment. My EP took one look at the graphic display and agreed that I was back in AF, at which he agreed to ablate me again a few months later.

Cost is going to be a factor for some. Utility for others. Do they work? Yes, they work. Are you wearing yours the right way? Is the back surface cleaned regularly? Will your doctor accept the information your device imparts? Personally, while I love the idea of a ring, I haven't worn a ring of any description for 50 years. Including my wedding ring. They get in the way of work I do where I have to use a strong grip or to handle materials and items that may mar or otherwise damage it. So, I prefer the Galaxy watch, which is complementary to my Android phone.

I don't know what 'inexpensive' means. For some, even $30 is costly, while others can blow that at lunch every day. That's life. What you want, most importantly I would think/hope, is outright utility and quality. That generally costs.