I run catch cans on my 2014 Mustang GT, my fiancee's 2007 Mustang GT and her 2017 Ford F150 with the 5.0. I have only seen the milky oil in the catch can on the F150. Mine and her Mustangs always just have black oil in them.
I first started running a Catch can on my fiancee's Mustang on the advice of a few people on one of the mustang forums. Her car was starting to blow blue smoke out of the exhaust on cold start ups when the car didn't have very many miles on it. When I took off the factory PVC line for the first time, it was soaked with oil inside. After adding the catch can, the blue smoke on start up stopped and has not come back.
When I bought my 14 Mustang new, a catch can was the first thing I added to it and have been running one for over 5 years on this car with no problems. Whether or not they help long term or not, they make me feel better anyway.