>>273759860
For the CV1 and Rift it's the same as piracy for any of the other VR platforms. 

For the Quest and Go it is risky as APKs get checked against what's in the store and what's attached to your account as a purchase. Playing a game you sideloaded onto your quest that isn't in the store? Absolutely A-okay famalam you go ahead. 
Sideloaded a game onto your quest that DOES exist in the store and ISN'T attached to your account as a purchase? Oh-oh spaghetti-ohs, you done fucked it now son.