There is the very practical limit of having to pay Epic if what you make with it happens to make you money above some arbitrary point. This introduces additional overhead to your finances - you may think it is justified, but it is still a limit.
Another very practical limit is that you can only distribute changes to the engine through either Epic's marketplace or a fork of their GitHub (so you cannot distribute changes through your own website, for example).
Also you cannot make a modification to the engine itself (even through the "allowed" means) and distribute it yourself as an alternative version/fork of the engine.
Finally, yet another very practical limit, is that you cannot take source code out of the engine for use in other engines and products - no matter what.
All these are practical - not ideological - issues that proper open source projects do not have. Unreal Engine is not open source, you just have access to the engine's source code for minor and project-specific modifications and even that comes with several limitations as i already mentioned above (and there are others tool, just read the engine's EULA).
Another very practical limit is that you can only distribute changes to the engine through either Epic's marketplace or a fork of their GitHub (so you cannot distribute changes through your own website, for example).
Also you cannot make a modification to the engine itself (even through the "allowed" means) and distribute it yourself as an alternative version/fork of the engine.
Finally, yet another very practical limit, is that you cannot take source code out of the engine for use in other engines and products - no matter what.
All these are practical - not ideological - issues that proper open source projects do not have. Unreal Engine is not open source, you just have access to the engine's source code for minor and project-specific modifications and even that comes with several limitations as i already mentioned above (and there are others tool, just read the engine's EULA).