Compressing a texture with ETC2 generates a warning: what do I risk?

In the editor, Unity has set the compression of my textures as "RGBA compressed ETC2 8 bits". But Under the Texture panel, I have a warning saying: "Only textures with width/height being multiple of 4 can be compressed to ETC2 format".

But it is just a warning, and the game just builds fine. So wat happens? Is the texture just left uncompressed, does Unity increases the size of the texture to compress it efficiently during the build?

And so, what should I do? Do I need to change the size of my textures to increase my performances/reduce my build size?

Thanks for your help!

(I know I should have used TexturePacker from the start, but now for this project it's too hard to go back...)