It is very difficult to describe what MITRE is. In my opinion, MITRE is a weird combination of government contractor, Government agency, and University. Most of the projects that I worked with were just simple engineering work, which you can find it in any other government contractor. MITRE has an internal research program, which MITRE brags a lot. This program is a highly bureaucratic, and proposals are only considered if they come from well-known/well-seasoned employees (employees with 10+ years in the company).
In MITRE I found 2 well defined groups of people; Young people (early career), and older employees. Young people come to MITRE right after school, and they only last couple of years in the company. Older employees come to MITRE usually from private industry, ready to enjoy the work-life balance, work for ~8-years, and then retire. In this company, life-work balance really means employees come late to the office, leave early, and give 75% every day. It is easy to get comfortable, and become obsolete fast. Some people like it that way, I can not judge them.
Finally, I would say that there is not perfect company. However, if you don’t belong to the 2 groups of people I mentioned above, think twice before joining MITRE.