I have no regrets to use Tamron 28-75 and Nifty Fifty on 5D.
The book I’m currently reading is about kids portraits. About 50% of pictures at this book were taken with 5D and with non L lens. No resolution issues at all.
That maybe the case but if the pictures are in a studio enviroment or taken with a aperture of f8.0 or above I'm sure they do look good but what i'm saying is that a FF sensor will enhance any flaws in a lens especially at wide open apertures.
Nothing wrong with a Nifty Fifty but as most comments say it does have its flaws at f1.8 and the 5D will use the full width of the lens and not just the centre like a crop sensor.