I currently have one under repair, and, having inspected it pretty thoroughly, I guess one of the reasons they're so light is the neodymium speaker (er...magnet), the other reason is the absence of a mains transformer.
The one I have here is just fine, only the speaker's coil has "gone south", seems like the output devices are rated to withstand the voltage rise due to an open load....
Though they seem well built, and they have an extremely favorable power vs. weight ratio, I don't like the absence of a transformer and the subsequent lack of insulation from mains. Looks like they revived some of the concepts used by Gallien/Krueger back in the 80s....those amps were extremely compact, but their reliability was rather poor.
This said, I don't want to judge them prematurely, I think they've been on the market for too short a time to have a dependable reliability record.....I guess time will tell ( My 44 years old AC30 is still going strong, but that's really from another era ), let's wait and see....