In February of last year (2021), I began looking at what is now my current boat (GW 306 Canyon). It is equipped with twin 2018 Yamaha F350 ("C" generation motors).
I asked the service manager at the Yamaha dealer and the owner of the dealer that was servicing my prior boat and motor (2012 Hydra-Sports 2300 CC with a Yamaha F300) about the F350s. The service manager said to avoid the "A" generation motors. Both the service manager and the owner said that the later years of the "B" generation and all "C" generation are fine motors. They all have the flywheel issue, but it is not a big deal and that the flywheels simply need to be replaced when the engine is operated for a certain number of hours at/between a particular RPM range. It's not that you have to stop using the boat immediately; you just need to get them done soon after the service message pops up on the command gauge.
From a performance standpoint, I am very impressed with their performance. They push my 2018 Grady-White 306 Canyon like a freight train. The boat barely slows when hitting big waves. They "cruise" the boat at 32 MPH at 3700 RPM at 1.4 MPG (30 GPH). They keep the boat on plane at 25 MPH or so and push it to a speed of around 52 MPH at 5400 RPMs. I have not run the boat at WOT. Currently, I am at about 200 hours. Time will tell how well they treat me.