It struggles a bit on certain types of workloads like hot keys, think heavy hitting a single sorted set. It's a cool architecture.
(I'd imagine implementing mechanisms for resilient storage and larger than memory data sizes would be the hard parts)
As good as Redis is, modern computers with multiple cores potentially leaves a lot of performance on the table. Garnet seems to be a well designed multithread KV-store (though sadly the benchmark page doesn't list benchmark for more complex objects even if the simple cases looks good).
Nowadays your webapp is often a persistent, multi-threaded web server where you can cache whatever temporary state you want far more efficiently than reaching out to a KV store.
Using postgres and caching common results becomes a non-issue even at scale. The main area where a super fast KV store shines is when you need to share a very large number of keys, where many servers only need to access a random subset and isn't interested in maintaining the full set and getting updates on a bus, where the values often change and needs to be revalidated so caching is ineffective, and where persistence is not needed so a database doesn't matter and a lighter KV store is acceptable.