If you mean emissions wise, that’s really going to depend on the bike. Old two-strokes rocking carbs? Yeah, they’re terrible. Modern fuel injected four strokes with cats? Pretty good, actually. And they get better mileage than most hybrids. I get 60mpg on my 900cc Triumph, which is a mid-sized bike by American standards and a big bike by world standards. Smaller bikes and scooters can get over 100mpg.
This seems both awesome and dangerous. The two analogies that come to mind are home canning and home brewing. They’re both generally safe and easy. But every so often someone gives their family botulism.