I would say that those who have a long track-record and advertise on the forum are safe, such as Bitvest.io. But there are others as well. I don't think any are significantly safer than the others.
Based on my research, Crypto-Games, Bustadice etc are some popular, safe and reliable websites that offer the option to invest in their bankroll. Do your research before investing op.
The three gambling sites above are currently highly recommended for investment...
Before closing, Yolodice became one of the prima donnas for investment as well, but there are reasons that the developer cannot explain so that they have to close their business.