The only good thing about WCF is you can easily add a service reference in Visual Studio. Which lets you immediately proxy it to JSON with Web API: https://github.com/jpsingleton/Huxley :)
I used to use ServiceStack and it was great although I hear even StackExchange are moving away from it now (https://blog.stackoverflow.com/2015/05/stack-exchange-podcas...). BTW use the StackExchange Redis client over the ServiceStack one as it's not properly thread safe.
Which works very much like a DB Connection, where the Connection Factory is thread-safe (and what's used to resolve connections) whilst the DB Connection instance it returns are not.
Sorry, I know you worked hard on it but this comes from real experience. We had an absolute nightmare debugging strange issues in the cache and traced it to this. Probably an older version though.
Each threading issue reported ended up being traced back to sharing a RedisClient instance across multiple threads, e.g. by using a static RedisClient instance, registering RedisClient in IOC, etc - i.e. instead of using one of the ThreadSafe connection factories. If you have any repro showing any multi-threading issues with proper usage we'd love to hear about it (https://github.com/ServiceStack/Issues).
Which works very much like a DB Connection, where the Connection Factory is thread-safe (and what's used to resolve connections) whilst the DB Connection instance it returns are not.