I will explain why REI support is needed and why it is better:

1. Better FPS and setting when inventory is open with items on the right (since there is caching)
2. More convenient and smoother in recipe views
3. Does not break settings with a large interface (see screenshot)
4. Flexible settings for displaying recipes and setting up showing / hiding recipes
5. Better support

If REI is installed:
settings GUI.

I don't have any plans to support rei.
I am not against it. I just have no interest.

If @Meduris or others of the team want to add support for it. I wouldn't stop them.
But it is highly unlikely that we will do that.

This is more of a addon material.

I personally like Jei enough where I can say I will keep using it.

And so far if there was major issues Jei has addressed them rather quick.
(And yes I reported a few)