Hydradrives - why the sudden increase in interest

[Graeme]

I guess the question is in the title but I'd like to know the advantages from those who have tried them. I seem to remember them going out of fashion post Kinwald's Worlds 97 win as they never were fitted on the XXX series (although I understand you can). Now they seem to be cropping up all over the place. Is this a brushless thing, a mid motor thing or just a 90s throwback?

Cheers

Graeme