wouldn't give you $.02 for a johnnyrude. I've had a bunch of them over the years, mostly 9.9's and 15's. The only good thing I can say about them is that you can get almost (almost) every aftermarket part to fix them. OEM parts are going away slowly. Note that aftermarket stuff isn't always a 100% direct fit.
Have had one or two mercury motors (mariner included). They are just different in a lot of ways. Once you get used to them, no biggie. One thing that stands out in the Brunswick (the parent company of merc/mariner) is Mercosil blocks. Google it. I've been a victim of mercosil and it didn't set well with me to have to buy a new block. Basically the cylinder bores are aluminum but plated, and the plating can come off. 25hp only I "think" but I could be wrong.
Nissan ain't bad neither is Tohatsu. You give up some things though. They're reliable but where I felt they fell short was in comfort. Short tiller handles, noisy operation, etc.
So with all that said I still prefer yamaha. Reliable. comfortable. Ergonomic. Some of the really old stuff, parts are getting harder to get but we're talking about older stuff....1970's and 1980's. Generally speaking if you need internal engine parts, the cost to repair exceeds the value of the outboard and at that point folks generally get another motor altogether. Yamaha built a few engines for Mariner in the 1980's as well, and then there was a partnership in the 1990's and early 2000's where Mercury/Mariner and Yamaha got together for powerheads on some outboards mostly 4 stroke stuff.
on 2 stroke stuff just be aware that the 2 main causes of engine problems are (1) forgetting to put the correct oil mix in the fuel and (2) mud dauber nests that are built up from sitting. Outboards don't have air filters. Mud dauber nests are just dirt. They build their nest and you fire the motor up the nest falls apart and gets pulled right into the engine. Won't last long like that. For that reason whatever outboard you are looking at, if you see evidence of mud bug nests on the engine or under the cowling, don't spend a dime on it-find another one. Rebuilding a powerhead just isn't economical. On the oil mix, you never know who or how they've mixed their fuel. All it takes is one tank of "lean mixed" fuel to cause internal damage, which may not be apparent when you initially buy the motor, but may swarm later on. Thats the main reason I don't like 2 stroke motors.