What are the cons of using ETC2 on iOS, or across all platforms?

We work mostly in 2D content on iOS and Android. We would like to avoid dealing with different compression settings for different platforms.

We like the file size, flexibility (Non-POT), and resulting quality of ETC2 compression. We think we understand the downside but just want to make sure.

We are aware that ETC2 is not supported on OpenGL 2.0, which is still in use on many devices we need to support. However textures do still work and show up.

Do we understand correctly that the reason it works is that the texture is decompressed on the CPU before it is sent to the GPU?

Is it correct that the texture on the hard drive is stored in the ETC2 compressed format, and only ever exists in decompressed state in memory?

We understand that this can result in slower load times, as well as greater memory usage, but are willing to accept these if it keeps our installed app size lower.

Are there any other downsides to using ETC2 everywhere?

We've been struggling with iOS install sizes being exponentially larger than Android sizes. Does using ETC2 have anything to do with file size differences between iOS and Android?