I've not looked at their site, but there's no excuse for such a site not to offer a proof-of-solvency, so its clients can be sure they're not investing in a Ponzi.
Exhibit #1 (cyberpinoy is the owner of StakeMiners):
https://forum.gethashing.com/t/pos-mining-stakeminers/3925/64
To be fair after some pressure they released their wallet addresses but that doesn't help much since it's still a black box from the investors' POV - they put BTC in and get some BTC out so this could still be fractional etc.