Those are good motors. I had a 1981 25 also… got it out of the garage of a partially burned house! It was so black from the fire, it took a major cleaning to see what color it was. I ran that thing for 15 years. Finally sold it as part of a package on a Jon boat. I seem to remember something about that particular model being a larger cu. inch displacement than others? I remember that motor was a screamer.

Yep, these are rugged, reliable, and easy to service…
Is this a salt or fresh water engine?
Don’t expect it to idle as nicely as an old 18hp, due to its funky recirculation system that prevents unburned fuel/oil from being dumped overboard like on older models. Nonetheless, it will idle all day without fouling plugs.

Very accurate criticisms indeed. That stupid electric primer is not easy to operate/understand…People don’t realize that the electric primer is NOT a pump, simply an electric valve. So, if you do not build up fuel pressure first by pumping the fuel bulb, a cold engine will be hard to start.
And yes, you usually had to "tickle" the primer after intial start, which is tricky to do with the key switch. But, the other fault is that these engines are extremely "cold blooded" and require a good deal of warm up before they will run acceptably, just like you say.
The idle and mid range running quality on these engines has never been good. I blame this on the stupid recirculation system that seems to favor one cylinder, making them impossible to dial in a good/smooth idle without an occasional sneeze, you have to leave em a bit rich.
This condition is only amplified when used on pontoon boats where low to mid RPMs are where the engine is mostly used. You could try to alter cam pick up a bit, perhaps a different pitch prop, but this usually only just moves the rough spot to a slightly different RPM…
So, in this case, the OMC tech was giving you "the straight scoop".

Yes, the primer complaints apply to all the engines…Most of the OMCs are/were cold blooded, so it takes a lot of tickling to get the thing warmed up…Unfortunately, most owners just raise the warm up lever and let the thing scream in an effort to warm up…The electric choke does seem a whole lot easier for owners to understand and use for sure…Maybe the electric primer was a "cost savings".
The pressure relief thermostat system works pretty well, but dirt/debris can get caught under the thermostat resulting in a cool running engine at lower speeds. And lord knows, you sure don’t want to have to do this job on the 32CI engines where the whole head cover has to be removed. In later years, even this got "over engineered" with all sorts of springs, cups and diaphragms.
Frank, your is a 1984 35hp, correct? I always liked the 1984 Johnson grey color and graphics, guess I am in the minority here….Always thought they ran better also! Yours had the simpler carb meant soley for these engines with just the one needle valve. In later years, in an effort to simplify parts usage, they started using the crazy carbs with the intermediate air bleeds and adjustable low speed needle (same style as the larger two/three cylinder models) on the 32CI engines.
And, like on the larger engines, if the recirc system isn’t working properly, I’m sure fuel/oil will puddle in the lower cylinder aggravating this condition…