future_syntax is targetted at people who develop using a recent compiler but want to make their software available to people using an older compiler. If you develop using an older compiler, then you are effectively not going to get a very good experience. Thinking about it, maybe the name is not so clear and should be something like syntax_shims instead 
Releasing support for let+ before 4.08 was even released was a bit premature and effectively the whole ecosystem is not yet ready for it. However, we were really eager to have a let-syntax in dune because we use a lot of applicatives and monads. And we can’t use ppx in dune. So overall we were ready to give up merlin for a little while.
Once 4.08 has settled in and 4.08 is what most people use for their day-to-day development, future_syntax will no longer get in the way since it will be a no-op.
You are right that the lack of integration with ppxlib is a deal breaker if you use ppxlib. I have plans to develop future_syntax and integrate it with ppxlib. However, it is a non-trivial amount of work and we already have a packed todo-list, so it will take a bit of time.