The power ratings on the wall warts are usually in the “fantasy” range. A wall wart rated for 3 amps for example might possibly deliver 3 amps - for about 15 minutes before some internal component fries. I never consider power ratings on these things as anything more than “somewhat advisory”
It is impossible to use a plain volt meter to measure for proper output. The only way to do it is bench test with a voltmeter, an ammeter, an oscilloscope and a variable load. Wall warts will do strange things under load. Voltages will fall off as load approaches max rated current, or output that looks DC smooth on the scope at 15% rated power will be disgusting with all kinds of square waves and spikes at 70% rated power. And this is assuming the wall wart is “working” as defined by the manufacturer - ones that have failed due to age, or shoddy components are all over the map.
You also have situations where a wall wart that supplies noisy power will work with one device and not another - I’ve had wall warts that generated power that was so bad that it would interfere with the CPUs in the devices, the wall wart would work for years with 1 device with no problems then cause another device to lock up - because the other device maybe used a faster CPU.
Unfortunately all these things are made in China and there is no branding whatsoever in that industry, no way to trace back to any specific company whether one is better than another, you just gotta keep trying different ones. That is why I collect them I have a cardboard box with about a dozen different ones. I carry a pocket volt ohmmeter with me and when I find a wall wart at a junk store that is cheap (as in $3 or less) and is a popular voltage (12 volts) and supplies 3amps or more I’ll test it right there and buy it if I get 12v out of it.
One of these days someone needs to build an Arduino circuit and program for testing these things, it would be easy - just ask the user for the rating off the wall wart, then when plug the wall wart in, and plug the power outlet of the wall wart into a circuit connected to the Arduino which then runs several load patterns against the wall wart and sees if the output voltage and current properly tracks the load.