V1 zigbee devices are not compatible with SmartThings. Most zwave devices and V2 zigbee devices will be compatible.
As far as your questions, you're going to get a skewed result, as people who changed over from Iris, tried SmartThings, and didn't like it will probably have moved on and will no longer be participating in the forum.
But you should get some answers from the people who switched and liked it, and that information may be helpful to you.
Every system has its own pluses and minuses, so different things work for different people. If you want really complex rule structures, SmartThings offers much more. SmartThings' greatest strength is its versatility.
One of the drawbacks is that it just hasn't been reliable. When SmartThings works well it is absolutely my favorite home automation system. But since last November I have yet to go 10 days without an impactful SmartThings failure.
The company is very aware of this, and a couple of months ago the CEO posted in the forums that they were making improved reliability their top priority. But they aren't there yet.