Here are two 6' bears I have taken with mechanicals. They work just fine, as long as your equipment has the right amount of energy and your bow is tuned well.
2002 Bear
2003 Bear
Most issues with mechanicals are due to poorly tuned bows and poor shot placement. Using a fixed blade will not cure a shot in the brisket or in the front shoulder blade, it also won't penetrate well if your arrow is flying sideways and fishtailing on the way to the target.
Mechanicals are bashed on a daily basis, but poor shooting skills are usually the reason to blame. Blaming equipment is so much easier for people.