I think the other issues are probably more important to be addressed before anyone should consider trusting their money with this type of service.
I don't have to trust them, but they are just one of the layer two solutions for Bitcoin, and I could also say that I don't fully trust Lightning Network or Liquid.
Idea in Mercury is that you have full custody of your coins at all times, and you could always withdraw coins to regular Bitcoin wallet.
Lightning Network protocol has measures in place that specifically prevent the theft of bitcoin by your channel partners. As far as I can tell, there are no similar measures in place with Mercury protocol.
I don't think it is accurate to say that you have full custody of your coins at all times, or at least this would not be true if you were to say "sole custody". This is according to my understanding of how Mercury works.
Every centralized service that holds custody of your bitcoin says that you can withdraw at any time. So did most ponzis before they imploded.
I don't think it is safe to entrust your money with Mercury until the issue of an malicious SE stealing your money is addressed.