UNC had a 2-year cert when I interviewed, and my med program had a fellow from there. (Note: this was all 3 years ago) From my visit, and the info i got from the fellow, the only real downside was the workload. Everyone really liked the overall attitude and interaction between faculty and residents, and felt it was a very supportive place, wherever you wanted to go.
I enjoyed the visit, liked the faculty. Ultimately, I ranked it lower in my top 10, and wound up at a place with great name recognition, but ultimately probably an even more rigorous workload.
UNC seemed solid. But the fact that they have yet another 2-year rating......it would give me pause
That being said, I also saw my home med-school prog get a drop from a 4 to a 3 yr rating for total BS, which was all based on politics. So...A 2 year rating may, or may not, be indicative of an issue. You need real opinions from current UNC residents, given anonymously, both those that have totally bought in to the program, and the malcontents (if there are any), to find the truth in the middle.