The cotter can wear out. The cranks can wear out. The cotter can wear out, causing the cranks to wear out, ruining the cotter and the cranks. They're usually heavy. They're usually on the cheapest bikes (unless you go waaayyy back into the 1950s, then they were on the best bikes too).

When they're in good condition, they work. When they're in bad condition they wobble, have slop, make noise, etc. In my opinion, worse than one piece ashtabula cranks - unless you happen across cranks in excellent pristine condition.