For those who are not familiar with CAT rating requirements, from Wikipedia:
...The required values can vary from 0.04 mm for single insulation CAT II, 50 V, to 28 mm for double insulation CAT IV, 1000 V. The exact values are defined in the international standards.[1] Such standards should be followed rigorously during the design process of the appropriate equipment.
So yes, the fact it does not meet CAT IV standard is pretty clear.
But I wouldn't go so far as to discredit this meter altogether. Having the ability to measure power is one of its main selling point and most electricians only do mains related work and would not need an accuracy higher than 0.1 W.
Except that you DON'T want to use this on mains.
I agree that it's a piece of crap that shouldn't be used. But let me play devil's advocate for just a moment...
We all plug in various devices into mains -- lamps, power supplies, radios, computers, kitchen appliances, etc. The only things which I commonly use that have proper HRC fuses are multimeters. I've seen some junk inside of consumer goods that make that awful UNI-T look fantastically safe in comparison. Yet most consumers survive their appliances. Most general consumer goods that most of us plug in to mains are generally safe, yet they don't have CAT ratings, and they're not as rigorously designed as a top quality industrial multimeter. What's the difference?
One difference is that meters are often being held in your hand as you stick them in the socket, so any explosion is right next to you. Another is that meters may sometimes be used for probing around in the main power distribution panel, where very large currents are available. But a big difference is that it's up to us, the users, to connect the meters to mains properly every single time. And most meters have both a high impedance mode, for voltage measurement, and a low impedance mode, for current measurement. The mistake that is too easy to make, and potentially deadly when made with a poor quality meter, is to probe a high energy voltage source when your probes are plugged into the current terminals. There aren't many other devices which we regularly connect to mains that have this potential for deadly user error.
That mains dongle that comes with the UNI-T meter would seem to prevent the human error of mixing up the voltage and current probing. That's worth something, in my book.
That dongle would seem to turn the Uni-T meter into a specialized mains power consumption meter that is almost as idiot proof as the P3 Kill-a-Watt meter (common in the USA). The Kill-a-Watt does not claim a CAT rating.
But even with the dongle, the UNI-T doesn't stack up well against the dedicated power meter. The UNI-T costs nine times as much, has a worse resolution of 1 watt versus 0.1 watt for the P3 Kill-a-Watt meter, and the Uni-T doesn't measure cumulative energy over time, while the Kill-a-Watt does. The Kill-a-Watt is a $20.00 specialized device for measuring power and energy consumption of mains-powered devices, not a general purpose multimeter. Still, if you want a power meter for mains power, you could do worse than buying a Kill-a-Watt for that particular task, while buying a general purpose multimeter that doesn't make a false CAT claim for your other tasks.