I've played with friction drive for years, using Staton, then BMP which I preferred because of easy bearing and roller replacements. My engines ranged from Robin/Subaru, then one Mitsubishi, twin Mitsubishis, and finally the GP460 4.25+ hp engine. The best in terms of power and reliability was the BMP with 2.2hp Mitsubishi engine. I was never able to fully harness the 460's power. It has a very small power range; go below that and the engine was a dog. Other times, its awesome power chewed up tires quickly, and the rubber dust was a problem.
Presently, I'm using Scooterguy chain drive for less than 2 months. I have used the 2.85hp Tanaka 47R engines and the 4.25+ hp GP460 engines. Still experimenting. The 47R is VERY loud at higher rpm. The 460 is loud, but not as loud as the Tanaka.
There are many issues I've had to deal with, like chain falling off the front sprocket, chain breakage, oil spray onto everything aft of the engine, "hunting" sprocket gears.
HOWEVER, the moment you powershift through 8 speeds, you will be hooked forever. NOTHING comes close to that feeling, especially if power and acceleration is your goal.
My present and best ride is my Diamondback bike with dual disc brakes, GP460 engine and 11t/25t rear cassette. My next build is a 29" bike with Scooterguy drive, either Tanaka 47R or GP460 engine and NuVinci rear hub.
I'm not into top speed; I just need to keep ahead of traffic going my way.
FWIW, I will never go back to friction drive.