Its easier to maintain constant speed over time (even if it feels harder than outside), and therefore very precise, controlled intervals.
But I have the same experience as you have. I think its alot harder to run as fast on a threadmill than the equivalent speed outside, and I am pretty sure I will be able to run the same distance faster outside than on a threadmill. I have assessed the threadmill vs outside performance, since I have friends with the excact opposite feeling who perform alot better at a 4x4 on threadmill than outside.So I guess motivation plays an important part in where you'll be able to perform the best!
Starting up with cycle two today due to sickness (agian, it's hard to have little ones in kindergarten!), and because of a vacation next week I'll have to improvise a bit with including cross contry skiing this cycle as well.
Planning to do the 5x2ks on threadmill and the 6x1k along a mountian road if I can find a long enough flat portion.
Luckily spring is comming, and hopefully the running possibilities will improve by the day in the months to come.
Aiming for mye first(!) 10K race in the end of april, so I plan on completing two full cycles before I hopefully reach my first goal of the season. (Never completed an official 10K)