Pretty much this. There’s a lot to like about Unity, but having used it professionally for 6yrs or so before UE4, I would never consider a major project without source access due to this reason alone.
Whether you have the knowledge to use the source is irrelevant, because you have the option to learn. That door is closed in Unity, and sooner or later you’re designing your game around the engine, where with source access you least have the possibility to re-tool the engine to fit your game.
I can’t imagine not having that creative freedom(Well, I can, that was Unity, and it was suffocating).
Also(directed at others), indie != amateur; plenty of indies with engine programming experience, what do you think indies did pre 2010 or so?(we generally made our own engines…)