Vantaggi
They have been owners of projects for a long time and maintain good relationships with clients.
Job stability (while other companies are laying off employees, this company hasn't even considered it).
Less stressful atmosphere compared to larger companies.
Svantaggi
Salary increases are extremely difficult to obtain, and promotions are almost nonexistent. Basically, unless you are a key employee the company can't do without, you have no leverage to ask for a raise. There's a high chance you'll leave after 2-3 years with the same salary you started with.
If you're used to working on more or varied projects, you'll likely get tired of the project you started on from day one, and a strong feeling of stagnation will set in. You'll need to take on personal projects outside of work to avoid plateauing.
Since the same projects have been ongoing for a long time, project management, documentation, and procedures are outdated and cumbersome, with no improvements because the team leaders are used to doing things this way.