It's not about the databases, it's about the migration in the first place.

If you have a problem that can be solved best with a graph database, then there is no problem. Many problem can be better solved with a graph structure. Choose one, and you'll be happy.

But, if your use-case is migrating from MongoDB to a graph database, that's a bit of a red-flag. What data model do you have where you can migrate from a document/schema-less system to a graph database? Maybe the tech lead figured out that a graph model works better for your data. If that's the case, then great -- migrate away.

But given that they want to go from Mongo to a graph DB, the fear is that this is someone who is only chasing the next cool technology and not solving an underlying business problem.