Ah, a tinkerer! I got my equipment for under $300, using a PID controller, Rice cooker and FS vacuum sealer, all new.

I think it depends on a few things but with my set-up it occasionally goes 1-2 degrees over but it stays quite remarkably on the nose for the most part. And I don't know that absolute precision really matters so far. With eggs, of course that can be one where you don't want +/- 5 swings but a degree or two isn't bad and it isn't sustained long enough to cause the food to get too hot.

My controller probably is measuring sub fractions of degrees because it seems to detect trends in heating/cooling pretty quickly and adjusts it's on/off sequence. So by the time I see it's +1 degree, it's already lengthened the cycle and letting things cool a bit. So no, the 150 isn't a mid point it cycles around, it pretty much stays there spot on for the most part.

The precision would probably be worse in something like a crockpot. If you go look at the controller site I linked earlier (Auberins) they have a PDF about the best devices to use and why. It might be an issue with your hot-pot, rather than the controller.

I've also heard of people putting aquarium bubbler in their cooking devices to simulate the circulation of the professional devices. But it seems like my current setup works just fine. My only wish is that the water bath were bigger so I could jam more stuff in there!