Here's someone doing exactly that, synchronizing a medium-sized hydroelectric generator to the grid.[1] The valve-to-turbine pipe delay isn't a big problem. This stuff is adjusted on the scale of tens of seconds. Opening the valve makes the whole reservoir slosh for a few minutes, and the operator had to wait that out before trying to achieve sync by hand.

[1] https://www.youtube.com/watch?v=xGQxSJmadm0

Thank you. Links like that are why I read HN. That video is totally unique.

Thank you posting this! Awesome video. As an aside, I went down the rabbit hole on this topic last year when I was considering building my own local grid composed of multiple, heterogeneous generators and solar. Instead, I ended up buying a pair of parallel generators and called it a day.