I recently chose to give up on Ogre3D as I was completely frustrated by my inability to make it run on mac. It's also way too big (I don't know if OOP is a reason for that, but I still tend to dislike OOP nevertheless). For example it doesn't seem that you can choose to not use the scene graph.
I loved that engine at some point, but as time passed, I wasted an incredible amount of time trying to either build it (oh the pain) or make a project run across platforms (mac especially).
I recently found an opengl 3.3 core profile tutorial (learnopengl.com, it's really good for motivated beginners), I felt really better learning that since I like to understand how lower level stuff works and how to make things as simple as possible.
Unity/source/unreal engine are great when one wants to sell a beautiful, smooth 3D game quickly and doesn't have too specific or uncommon requirements. But when you want to experiment, there's nothing else than raw opengl.
I really wish Ogre3D was lighter, or that there was be a minimal engine with plain helper functions that let you use, understand and talk to OpenGL to make the best use of it. I can understand that indie devs want to be able to use the best 3D right away, but sometimes it can be nice to give more control to the programmer.
I guess I hard a hard time learning by myself, or that I'm just trolling out of frustration. 3D is not easy and sometimes I think some small dose of KISS would be appreciated.