I've purchased an older 400mm f6.3 t-mount lens and I'm considering buying a Helios 44-2 2/58 M42-mount lens. I've used the 400 for a week on my Sony a6000 and I don't see any problems with my copy, outside of me failing to manually focus correctly. The images look fine and the boken isn't the best, it does work. I know that manually focusing a lens is an art that will determine the sharpness of an image so I'll improve that over time.
Is there any downsides of using older lens, specifically the ones I mentioned, in terms of image quality compared to say a modern Nikon 400/2.8 or Sony 55/1.8? (Ignoring auto focusing and EXIF data.)