File under, "not sure if a very good joke, or serious".
I'm leaning toward the former. "New tech lead" is the give-away (or is it?).
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.
To be fair to the teach lead, I do feel like it was the other way around. MongoDB was foisted on us on a new project (we were previously SQL) by a software architect who left soon after. I've never felt that MongoDB was a good fit for what we want to do, but I want to return to SQL.
