there are more similar projects like this, like IceRock mining. They promise 439% ROI per year - https://icerockmining.io/
That doesn't seem to be realistic, however why are there such a big differences in ROI calculations between Hydromined and IceRock? Is IceRock just an un-realistic scam, or is Hydrominer just less efficient?
My personal opinion? Never trust any company that promises returns that big, you can't promise returns, you just can't. Not in the mining nor trading space. Ask if they have any proof regarding their past performances (and if they can back that up with the transaction from the pool).