I have spent a lot of time in CO - living there and returning there; but on scale of 10 as to knowledge I am probably about a 7 or 8. As are others on here for good, solid, honest, no bs info. There is one exception to that rating scale - it is 4runnerguy; I give Ken a 9.9 (nothing /nobody is perfect)! :-)
As a note here - many of us don't mind mountain driving driving to some degree. We have done it a lot and know the parameters. What has to be taken into consideration is where the person is from that is asking - this Forum or anyplace - and their level of personal comfort with it and also their experience with it.
It is not just the road width, shoulder width, guard rails lacking, dropoffs, hairpin turns requiring two lanes with your rig, elevation difference (up or down), rig size or capabilities, etc., it is the fact that you have to deal with others who have the same issues. Many times those - and also others - who feel they are owed the half out of the middle and it is not fun nor safe dealing with that.
Or they are scared to death being their (probably did not ask or realize the danger, etc) so they drive 10mph - people on this Forum have ever recommended others to drive slow; this is one of the most dangerous things people can do as it causes irritated drivers and accidents.
Sorry I digressed a little but roads leads to the comfort and safety issue.
You will note he says this is his first time driving a rig thru the mountains.
BTW, I have many times driven these roads being discussed here.