As a colorblind player, the 'at war' glow is very nice. I have a lot of trouble distinguishing between civs, between city states, and frequently even seeing cultural borders compared to open ground. Being able to tell at a glance if that nearby unit is one I'm at war with is super awesome, and this is the first time I've ever been able to. Saves me a ton of mousing over, and accomplishes a goal that 'unit owner' overlay couldn't.
Providing another perspective. I wish mod devs would focus on quality clean code and cross-compatibility, especially interface mod authors. This is a good feature for me. Those two opinions aren't contradictory.
You made a good point. Though as non-colorblind player, I was fine without "outline", since you can achieve everything at glance, with strategic view, without ton of "mouse over" which is quite persistent with EUI, that's contradictory to "saves me a ton of mouse-over. Again, this is just personal opinion, I am not trying to enforce anything.
That, said all to it's own, people are going to like or not feature but in bigger picture that doesn't matter, because we are all force to use it.
Cross-compatibility between mods (though this , more often than not, is not possible or worth effort) and modularity/customization is something that's more manageable and preferable to end users, for instance ability to disable latest unit outline would be appreciated , from people who don't want it.