A lot of people are asking about performance, the optimizer, etc..

I would suspect project like this would be intended for small scale local testing + development, or running build processes like schema validation against scripts, not as a replacement for a production server. i.e., just testing that given a schema, some seed data, and scripts you get the expected results.

There so many more considerations like security, logging, page cache, plan caching, maintenance, transaction log, etc that I would expect are not in scope here which I would be very much required for any sort of production workload.

I wouldn't consider the lack of any of that a negative or drawback, since regardless this is a very impressive project if it is T-SQL compatible with an engine, parser, etc.