I agree proper, effective and competent training is critical.
The trouble is training is one thing, many of the actual incidents that you train for rarely occur. Cops are human. "Riot police" are primarily routine cops from all various regular duties. Many of them normally doing routine tasks such as traffic enforcement, airport security, desk (administrative), investigations, on and on. Then abruptly they are positioned in extremely high stress circumstances and expected to perform flawlessly. It just ain't realistic. Many of them perform brilliantly. Some of them do not know how to measure responses (cannot think straight) under immense pressure.
Then they are put under a microscope buy their "officials", the media, and the public. Eventually leading many of them (even the best) to wonder "why the f**k am I doing this".
Reasons why the turnover rate, disabilities, etc. are so high. Also a reason why the cop suicide rate is one of (if not the highest) rates among any occupation.
Most people do not have a clue as to what they are up against.