I've done a lot of charger teardowns, and very cheap chargers always violate UL safety rules. It's a combination of cost (e.g. triple-insulated transformers), it's hard to fit clearance distances into a small charger, sloppy manufacturing, and they don't seem to even try.

I respect your superior expertise, of course, but looking through another site that has done systematic teardowns and measurements (https://lygte-info.dk/info/ChargerIndex%20UK.html), it appears to me that this might be too categorical a statement. While counterfeit chargers almost always seem to be dangerous, there seem to be some Chinese brands which produce quite respectable chargers, while still being fairly inexpensive.

The point is that you don't know. A legitimate branded charger will have been tested in a certified test lab, while an off-brand or counterfeit charge is likely to have a counterfeit UL mark. It might be perfectly safe or it might be a complete deathtrap, because nobody has actually tested it for compliance.

I need a bunch of chargers for a sensor network (Right now using Wemos D1 Mini boards). Any advice on optimizing for price without compromising on safety? I don't trust the sellers that sell $2 chargers...

Anker, Aukey and Blitzwolf make reasonably priced and high quality chargers.

This summer I, had a dehumidifier plugged in with a 3-prong connector . . good thing I, checked it shortly after I started the unit ,the adapter was very/warm . . a short in a cheap adapter . . went to electric supply store and made sure I got a high quality UL rated brand.