I think streaks are a good thing (consistency) if you push the user to look at them in aggregate (ala the Github green checkbox) not in terms of punishment for missing a day (aka a single number).

I like how Anki does it for example.

Also, guide the user to find a non-burnout rate. It is easy to set yourself up for destruction with learning apps and I like how Anki told me "slow down Cowboy" in terms of the new card rate because I hadn't worked out that going too fast on this would result in an avalanche in two weeks in terms of review cards.