The idea is very good - the tests approach is effective and stimulating; they also provide a range of products to study/experiment with. However, I think that the approach to the learning material is "love it or hate it".
By design, the service doesn't provide any documentation; it provides references to existing technical documentation (of any kind, including blog posts).
Those who expect a focused introduction to each topic will find it very tedious or hard to proceed (for example, the SQLite exercise has important details buried in a very large and confusing webpage), and likely hate it; those who like the challenge of understanding loads of raw documentation will love it.
It's more complex than that. People may need stimulation in order to follow through a certain learning process.
Providing a structured path and (automated) test means can be stimulating, and can make the difference between (deciding to) learning something or not.
Some people are certainly entirely autonomous, but at the very least, there is a spectrum of need for stimulation when approaching a topic of study.
AFAIK there are no other services that provide multiple languages, automated testing and team features, but if you know of any, it's certainly useful to report them in this thread.
Exercism is the closer service I can think of, but it's based on simple exercises, not real-world projects.
There are a few books that have a similar target (build X), but much narrower in scope (either a single language or pseudocode, and certainly no automated testing/team features).
I have finished their build your own Redis exercise. It is very well organized and there are hints/discord groups in which you get sufficient help if you make the effort.
This model is great for someone who loses patience with all the groundwork setup.
However, I do agree that the next leg is taking this MVP/Prototype level to production and ideally sell it as a real alternative to the commercial version of Redis.
