re hydrotherms. I think their use is more popular across the border. The reason I've seen stated to discourage their use here in the US is that there is no way to standardize/ calibrate them. If it works for you, then use it and spend your money elsewhere as you're only making syrup for your own use.
The one concern I'd have is with what someone else mentioned earlier about them containing mercury. I doubt any new models contain that but if yours is older and you think yours does then don't use it and dispose of it safely as hazardous waste. Mercury on the loose would make any syrup it contaminated unusable and you'd also have a big biohazard on your hands. Mercury contaminated syrup would also need to be handled as hazardous waste and would need special disposal. I can't believe we used to play with the liquid stuff as kids in science class..Times have sure changed!