1. Lack of mentors will be an issue.
2. "Jack of all trade, master of none" problem. This is not a problem at personal level. But this is a problem in an industry where recruitment happens for specific domain or language and not for versatility of engineers.
3. In this age, employees are treated as use-and-throw commodity. So not having a skillset that is transferable becomes a major issue if you had to all of a sudden look for a job.
The problem you have mentioned mostly appears in small contracting/consultancy companies that take projects of other companies.
Other issue to lookout for is not getting sucked into a project that uses some obscure technology that no one else uses e.g. having to work on some proprietary in-house developed language for all the projects.
I have seen all this happen at lot of software consultancy and contracting companies. The founders of these companies start pulling all sorts of project right left and center to make lot of money. But it is the emloyees whose careers are destroyed.