Under many circumstances an old uncoated lens will perform almost as well as a modern MC lens, it will all most certainly appear to be more tonal, lacking the wiry modern sharpness of Japanes lenses, but that's not a bad thing.
I have a couple of Tessars, 1919 & mid 30's both are very good, sharp with plenty of contrast, edge sharpness isn't quite up to modern standard but is only noticeable with enlargements. A late 30's Dagor, later factory coated is on a par with modern lenses.
Where older lenses just can't compete is shooting into the light, I'm just putting some image together for a project and there isn't the slightest hint of flare or degradation shooting straight into stage lighting, or into the sun with the best modern lenses.
Reality is if you use an old lens, remember how people recommended their use at the time of manufacture.
Then also think of those wonderful images you've seen that were made with these old lenses when they were first produced. If you can't match the quality there's something wrong with your technique