I think you did well to get the refund and the $70 voucher from Frontier.
On rare occasions that I use different carriers for a trip as you have done, I allow myself many hours connection time and I avoid Frontier (and other LCC's) as they have such a limited route network that any delay or cancellation can cause a huge problem.
UA has 6 flights per day non-stop DEN - LAS...
I am a big fan of Frontier. I fly with them many times each year, but only non-stops to LAS and Florida, and only when I can afford to be delayed a day or two in the event of a cancellation.
With probably 50 flights over the past 2-3 years, I have only had 1 delay of a few hours and 1 cancellation that got me home a day late.
Both times I received a $100 voucher
side note: Frontier vouchers are not nearly as valuable as other carriers. The person it is issued to is the only one that can use it, and they retain no residual value.
I tried to use one for myself and a family member on the same reservation, and it discounted my portion of the fare, but not the other passenger. Also, they have a short, 90 day expiration.