It is safe to buy SNGLS with our SingularDTV Lightwallet.  We are being cautious calling it a "Beta".  It is the same wallet tech that Jaxx, Metamask and lightwallet.io use.  We consider it "Beta" because it has not been out in the world and used/tested as long as "Mist/Ethereum wallet".  After it has been out in the world longer the "Beta" label will be removed, just like what happened with the Mist/Ethereum Wallet at one point.

I have been using the SingularDTV Lightwallet for several months.  I use it like a "hot wallet" and store larger amounts in Mist or in Icebox cold storage, but that is my personal preference.