Mercury is the only statechain implimentation as far as I am aware. 

The difference with a sidechain (like Liquid or RSK) is that you only verify the chain of signatures for the one coin (instead of all coins), it is not custodial and there is a unilateral exit (backup tx). 

tptrevethan

This is the same as any statechain, right? And what differentiates it from a sidechain? 

joda

bitcoin

Mercury layer AMA

One of the main motivations for the development of mercury layer (from the original mercury statechains) is the fact that the operator is blinded: they don't and cannot know anything about the coins (or even if they are coins) - it just stores key shares and updates them in accordance with the protocol. 

This does put the operator in a very good legal position as opposed to say, a custodian. But ultimately there is nothing stopping a government from doing whatever they want - and could make such a thing illegal (but IMO this would be like making cloud providers and KMS illegal). 

But crucially, even if the operator gets shut down, all owners get their coins after a timelock with the backup transaction. 