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IIRC there are some technical challenges which are currently blocking GA. 
Maybe <@U02J3T6A8LB> can say more. 

Yeh we still know there's some issues with this, but were also not yet sure how to solve those issues and we've been prioritising other work.
Having said that, despite being in preview they should be stable but you probably want to ensure any workflows you design around using them have an easy escape hatch to turn them off, in case you hit one of the conditions currently not supported (mostly things around `apply` but some providers do odd things with defaults as well)