I have found the same as you describe. We had waited for many months for help from ESRI, as promised, to understand intent - ex. reclaimed water, sewer and storm sewer questions – this to limit customizations as is our directive.
My understanding is the LGIM current state was the attempt at creating a simplified version based off some very sophisticated models – some round pegs didn’t fit in the simple square hole - we are now aiming inbetween (some customizations). Advice from ESRI is to customize as needed, but use aliases and whatever we can to limit actual model changes. We have brought many things to ESRI's attention and they have told us that their developers will welcome – one is separating reclaimed water from potable. There are many other obvious oversights in the utilities models and creation templates. We are noting these and communicating them to ESRI – many of these I would expect other Utility owners will be requesting the same or welcoming – ex. Air release valve diameter set to update to connecting pipe diameter in attribute template – depending on what detail you are collecting pipes/connecting stems, and within control valve this might occur in the simple model, but not enough to set rule - detailed pipes, then yes. Just to your note, we have made individual domains for most – using shared domains is not going to work well, ESRI is well aware of this one, and likely working to improve them. I did think there would be plenty of documentation as usual with ArcMap comparing to other vendors tools like Autodesk Map 3D (very bad, with lots of outdated help), but not so much in regard to LGIM. We still have a lot to work through.