In my personal experience, it did get a lot harder from 2014 to 2016. My first TN-1, in 2014, was approved very easily with no issue. Had all the standard documentation and there was no ask for additional evidence.
Renewing in 2016 didn't go as well... I was initially denied at the Port of Entry, had to get a lawyer to coordinate an appeal. That took about 30 days, but eventually was granted a 3 year TN. I worked with Peter Roberts who does AMA's here from time to time on the appeal. Actually can't recommend him enough to any Canadian's looking to work in the US!
FWIW - My lawyers said renewing your TN Visa, when you live in the States is expected to be difficult - as opposed to living in Canada, and crossing the border every day for work.
My company heavily suggested I switch to h1b simply because they expected my TN visa renewal to be denied (I am now on a h1b1)
I learned that my mistake was crossing back into Canada and doing the renewal at the Port of Entry. Apparently you can do TN renewals directly through the Department of State, while still being in the US. That's how we did it for the appeal and everything went smoothly.
Renewing in 2016 didn't go as well... I was initially denied at the Port of Entry, had to get a lawyer to coordinate an appeal. That took about 30 days, but eventually was granted a 3 year TN. I worked with Peter Roberts who does AMA's here from time to time on the appeal. Actually can't recommend him enough to any Canadian's looking to work in the US!