I may be mistaken but I think they were sold on Canyon MTB full builds for quite a while which caused a lot of confusion for others asking a similar question and being told they're not suitable for MTB. IIRC they work well on MTB despite Ergon not recommending it. Can't promise I've remembered all of this correctly though!

OK thanks for the info, think i'm going to take the risk and buy one.
I've read a positive review on Gravel Grinder News, so I don't think general trail riding is going to be too heavy for the seatpost.

I would have thought it much more of an MTB product, I would even try one might make me ride the hardtail more often.