The big negative that has been mentioned but IMO cannot be overemphasized: It is a bureaucracy. At the volunteer levels, many of them want to feel powerful in exercising the authority of their positions. (Hence the advice that CAP varies a lot from squadron to squadron.) Over and above this, there is a national full-time staff whose main output is ever-increasing bureaucratic requirements, especially for pilots.
There are steep differences between different cap wings, it correlates closely with the ratio of lawyers vs pilots in the wing ;-)
I have been member in two wings. Lets say, you want to have a sarex for members to advance their mission related qualifications.
This is how it works in wing 'A':
March: Super master above all training plan is generated by assistant associate wing training officer. Contains multiple training events and a 'plan' to create X number of new mission pilots.
May: Fractal order under super master above all training plan is issued to schedule a training event about a month out. There are strict requirements as to who can participate, if boxes x,y,z have not been filled by the right people, you can't even show up. There are required pre-meetings and signoff events, if you however show up for them, nobody really knows what needed to be done and how.
June: 2 days before the training event. Frantic emails buzzing around: Nobody has signed up, we need bodies, this is open for anyone, show up, ooh nooh. Btw. we dont have funding.
Evening before event: Not a single mission scheduled.
Day of event: After great confusion and with non-functional communications, a single mission is flown, but by the time the air-crew gets in the air, the ground teams have packed up and gone home.
This is how it works in wing 'B':
Tuesday: Hey peeps, aren't there some members who need quals signed off ? How about we use some state flying money to do a local sarex on saturday. The cadets will put together a ground team.
Friday: Showtime is 9am, bring a bagged lunch.
Saturday: Lots of flying, lots of training.