I am a dual US/UK citizen who has recently moved from the UK to Belgium.
When entering the US I use global entry, and it is fine (although lately as more people are learning about it, I sometimes have had to wait a minute or two for a free kiosk...)

In the UK I have used both e-Gates and IRIS.
I MUCH preferred IRIS because fewer people knew about it so there were NEVER any lines!!!
Other than the lines, no big difference between e-Gates and IRIS. It's quicker than waiting for a human.