WaniKani is a lot more limiting than Anki (which is free and open source) since it has a fixed schedule, while Anki allows you to learn at whatever pace you wish. In addition, it offers next to no customization.
There's a WK deck if you really want to use their content.
Next to no customization is what I want for something like this. I want to be able to mindlessly drill for a few minutes a day, not spend a lot of time fiddling with my decks to get them perfect.
There's a WK deck if you really want to use their content.