I've got both a 1973 Mercury 9.8hp and a 1974 Evinrude 9.9hp, while both are good motors, the Mercury is the more finicky when it comes to the carburetor. I've rebuilt that thing 20 or more times. The Evinrude just goes and goes. both have had new fuel pump diaphragms an a half dozen water pump impellers over the years. The Evinrude is the stronger motor but its also a heavier motor and likely a bit larger displacement.
I never heard that the '74-76 Evinrude 9.9-15 hp motors had any issues.
As with any long running model, improvements were made over the years but none were 'bad' motors.
The only Evinrude products I stay away from are those with electric shift, and Ficht motors, but those are all much larger motors.
The first two years of the 9.9 Evinrude has points, which I don't see as a problem, but that changed by 1976.
My 1973 Mercury 110 9.8hp was my first new outboard, so it'll likely be with me till the end, the Evinrude belonged to a fishing buddy back in the day, I run the Mercury in freshwater, the Evinrude in saltwater. The stainless hardware on the Evinrude makes it the obvious better choice over the Mercury for salt or brackish water use.