I think it’s just a lot of variation. Some people just use smartthings, some people use half a dozen different controllers.
Many people use a Phillips hue bridge and SmartThings: I would guess that’s probably the most popular dual controller set up.
https://support.smartthings.com/hc/en-us/articles/200848024-How-to-connect-Philips-Hue-devices
Harmony is also very popular, and the official SmartThings/harmony integration is the easiest way to add control of entertainment devices to a SmartThings set up.
https://support.smartthings.com/hc/en-us/articles/205653834-Logitech-Harmony-Home-Hub
Yonomi, like IFTTT and Echo isn’t a controller in itself, it just allows you to use events from one controller to trigger events from another. Quite a few people use it.
There is a thread for Yonomi, you can ask any specific questions there. 
Some people use multiple Z wave controllers. It doesn’t work perfectly, but I can give you some added features. Some People will also add in a Staples connect or wink hub just to get the Lutron Caseta integration.
So there are a lot of different variations. Some people want to see everything in one app, but others are fine with having multiple apps, specially for set ups where they don’t have to use the app very often because everything runs on a scheduler or from event triggers.
So just lots of different variations depending on preference and needs.