"Been there, done that!"
It's been something like 30 years ago, but I had to try one, you know? Got a ten meter 'Isotron' to put up at work. (10 and 11 meter, sort of.) That was one of the sorriest things I've ever had. I finally got it to tune, but it was a huge PITA! Talk about 'up/down/up/down/etc! I can honestly say that the comparison to a dummy load is a valid one. Except if a dummy load was that much trouble to use, I'd throw it away.
Take a look at what the things are. It's a tuned circuit, a coil and capacitor. One tuned it will accept power like any tuned circuit will, and it'll even radiate, sort of. But... and it's a big 'but', it don't work worth a hoot, and is not worth the money unless you absolutely can't figure any other way of going about it.
[The only tuning there is to the 10 meter version is by adjusting the capacitor thingy, and it is unbelievably 'touchy'. If a bird ever lands on it and leaves a 'deposit', the thing is no longer 'tuned'.]
'Isotron' has been around for what, 50 years at least. If the owner ever made a living from them, he had to really enjoy newspaper soup and grass salads. It's not a scam, they do work. But they work very badly.
- 'Doc