I've also read that blood pressure monitors that go on the wrist (or finger) are not very accurate. I'd get one where the cuff goes around the upper arm.
I took my BP monitor into the doctor's office once to compare it with the value the nurse got using the stethoscope and it was just about the same. So, at least for sitting/resting BP, it works well.
I do have a quick comment about measuring your BP at home as opposed to a doctor's office or a tilt tablet test. As others have noted (on another thread, I think?) if your pulse pressure is too low then these machines can have a hard time reading BP. Pulse pressure is the difference between systolic pressure (top number) and diastolic pressure (bottom number). If it is too low then many times these BP monitors will report an error.
I don't know how often POTS patients have low pulse pressure but in my case (NMH, Neurally Mediated Hypotension) it happens a lot. If I try to take my BP while standing (first thing in the morning, before any medications) then it is hit or miss. Once I got a value of 97/84 (a pulse pressure of 13, and an unusually high diastolic value, it's usually between 60-70) but many times all I get is an error. Because the automatic BP monitor simply says "error" I can't ever be sure whether it's low pulse pressure or something else.
That said, I do think BP monitors are useful devices to have. I bought mine many years ago (14? 15?) and it's still working just fine. It's an Omron, don't remember the model number. And, as you said, the heart rate is also measured so that might be helpful if you suspect you have POTS.