Is it ok to have just one scene for a whole game?

So, after alot of work were finally getting close to getting this game out, and after doing some research to some random things, I came across something called Application.LoadLevel.

I never saw this function before, and I started looking into it and realized that each scene in your project is considered a level. Now, I have my whole game in one scene, and use prefabs to spawn chucks of levels, since they are simple levels that all move to the same degree, similar to how a endless runner constantly spawns until the player dies.

So a few questions:

1. Is it bad for processing to just have one scene, and have the level load logic just Instantiating a prefab, with all of the ui in the same scene? Will I get a noticeable performance increase if I just load levels and not prefabs?

2. Besides realizing that nested prefabs can't be updated (yet), I see an advantage in using levels. I don't see it taking that long to setting up that logic, but lets say I have Scene_Group_1, which has a small level chuck of a level. Can I load multiple instances of it?