Depends on whether your goal is to efficiently learn a language or to outsmart a gamified ranking system to become rank one in a leaderboard.
The goal in spaced-repetition learning is to view a card just slightly before you’d have forgotten it.
- Wait any longer and the word is no longer part of your vocabulary.
- Show it more often and you do unnecessary work.
If you want to learn the language and the regular interval is too long, and halving the intervals means that the card is viewed closer to when it needs to be viewed, then that serves the purpose.
Nonetheless, this can be misused by people who only want to beat their competition in the leaderboard. Halving the intervals means they get to the point where a card gives 16 rather than 4 points quicker than people who play honestly.
I’m mature enough to not care when someone has more imaginary points than I have. But I can see how others would get frustrated because they’re beaten in the leaderboard all the time by others who don’t play honestly.
Although I believe, if your goal is to cheat, you can already accomplish that simply by changing the default values (1/10/30/180). You don’t need a “Hard” button to get to a card‘s 16-point-state quicker. Perhaps some people don’t know that and the “Hard” button will be obvious enough for them to cheat when they previously didn’t/couldn‘t. So it could be that cheating gets slightly worse but not by much, I’d bet.