My shooting buddy has an su16 that I've used a bunch.
First, I'm not a fan of anything KelTec. I do think they're plastic crap.
His first 1000 rounds or so were tortuous. It was piercing primers, ftf, fte, etc. Accuracy was less than poor. To hit a barn, you needed to be inside it.
It took 2 years to get a warranty bolt carrier, before it would fire with any sort of reliability.
Ergonomics, for a "black rifle", will leave you wanting. His wears AR furniture, if it matters.
All that said, it's has ~6000 rounds through it now, and it goes bang when it should. It's light and easy to aim, and still fun to shoot. (kills me to admit it)
Since he bought his XCR, the poor little Kelcrap has been collecting dust.