Wages are not the highest in the industry. You can earn more elsewhere. If you buy into the culture, projects, and long term prospects, then it's not a problem. If you need a higher wage right now then these reasons may feel a bit hollow.
Due to the shares and culture there is the occasional instance of senior staff appearing to wait out their retirement. As such they have been sidelined for further promotion and aren't as dynamic and involved as they once were. This can be frustrating if you come against this as you rely on them to be your spokesperson to the wider company.
If you expect to come in at 9am, do what you're told, and leave at 5:30pm don't expect to be promoted even as you get better technically. Some companies like this 'design centre' mentality But MM isn't one. You may be very good, but if you're not making the life of someone above you easier you won't get noticed.
Maternity and paternity is weak for a company with such a strong culture. I understand this is changing in 2017.
Being employee owned, with no outside investors is great. But it comes with certain restrictions i.e a reliance on organic growth and small acquisitions only.