I think you are lumping two distinct questions together that are wholly separate:
- Is WK effective? Does it fulfil its purpose of aiding the user in learning kanji efficiently?
- Does WK have negative impacts on its users?
The first one is what most people are answering here, and there appears to be ample scientific evidence for the claim that mnemonic-based memorization and spaced repetion are effective tools for acquiring factual knowledge. WK is more rigid than other systems because it’s fully productized. It makes executive choices to lift the burden of making those choices from its users, in the same way that, for instance, Apple makes executive choices about how iPhone apps have to look and function to ensure a consistent user experience.
What you’re actually looking for, though, is a discussion of the second question.
(Disclaimer: I’m not a doctor or psychologist.)
As ejplugge said, WK is employing several methods that are commonly lumped together as “gamification”. Putting a level number next to your username makes you want that number to go up. Most actions you take give some kind of reward, from guru’ing an item to unlocking lessons to leveling up. These strategies are widely used to form habits. Often this is done with nefarious or at least dubious intent, e.g. social media apps are specifically engineered for frequent randomized rewards to keep the user from engaging with the app more (which leads to more ads viewed inbetween posts and thus more money for the social media company); a fact which is not sufficiently disclosed to users. (Remember the outcry when it was revealed that Facebook turned its users into guinea pigs for a study that proved that a different feed algorithm made people angrier?)
However, habit-forming techniques are a tool that can also be used for good. Suppose that someone is a couch potato, but they want to get into the habit of exercising. If they can find a method that manipulates them into exercising, that’s a good thing in their book.
So I’m not condemning WK for using habit-forming gamification techniques. If they didn’t do that, I’d probably have given up on learning Japanese already, so their gamification is entirely in my interest. But when a habit-forming thing meets the wrong person with the wrong state of mind (for whatever reason), there’s a risk of unhealthy addictive behavior.
If I were Tofugu, I would try detecting usage patterns that hint at an unhealthy use of their system. If someone does reviews ten times each day for a long time, that looks like obsessive behavior and the system should try engaging with the user to advise them to give themselves some time to breathe. This is just an example. It would be a great idea to engage with medical professionals to gain insights into users’ mental health and discuss which actions to take.