No. 

I look at the threat level.  Then I look at the unit(s) stats and make an assessment.  Then I look at the threat level again.  Then I usually shake my head.

It'd highly inaccurate.

I care about HP, defense, and attack. The individual importance of those all depend on my own army's strengths and weaknesses. If I have a high-attack army,  my enemy's defense is less important to me.

A good rating system is not an individual index, but a direct matching of my army versus another's.

I imagine the tactical layer makes it very difficult to ballpark actual threat levels, because of what a player mind can do on a tactical grid that an AI can't, and the things the AI cant take into account, like movement and special abilities.

If I were playing an Ironman game in roguelike fashion, I would definitely pay attention to it, as in that situation I would be erring always on the side of caution.  In a casual game, not so much.

One thing I would like to see is a ballpark threat level indicator for ruins/caves/quest locations.  While the specifics should still be a surprise, If you want to me to accept the wound system  I need to feel like I can make a sound strategic decision on whether or not I can do something.  It's so random now, you can wander blind into these things and just laugh at the lethal BS that comes out of nowhere, flip the game off, and reload, knowing you never had a chance.

All those things change with the stack it's being compared to though.  I think about the best we can realistically expect is a ballpark evaluation of HP/ATT/DEF, unless you want the processor overhead of the auto battle simulation when looking at any stack... which still wouldn't be accurate, because it oversimplifies what can be done on a tactical map.

In the end, it's not really important to know how dangerous the game thinks a stack really is to a given army. We only need to know how dangerous other stacks that the game labels weak/ medium/ dangerous have generally been within a certain range, and expect those results to be fairly consistent.  The human can make a better decision about that at any rate, being able to more accurately gauge the strengths of their army.

EDIT: Wrong thread.

I upped the value of unit level's threat addition so it works pretty well most of the time. Level is the biggest factor right now with difficulty.