As I understand it their downsides come in a few forms - They are heavier than a derailleur system, which probably matters less with electric assist - They ge

As I understand it their downsides come in a few forms - They are heavier than a derailleur system, which probably matters less with electric assist - They generate slightly more friction in the mechanism particularly before being "broken in" - They are not as user end serviceable This combined with the price is probably what has prevented their mainstream infiltration in a big way (yet)