It doesn't look too bad. Seems like it is stretched tight and the squeeze looks like it may be just compressing the insulation.

 

Airflow won’t be compromised by that strap, but a slight amount of energy will be lost by the insulation compression. It will be fine.
Some compromises must be allowed on a good duct install.

 

Is there a reason that the strap can't be attached to separate joists (rather than to the same joist and same point)? This would reduce the amount of insulation compression at the top, which is also closest to the hottest part of the roof?

 

Catch 22.

On the one hand you loosen the compression on the insulation, on the other hand the hanger is helping support the stretch on the flex, taking pressure off the 2 ends. With the flex just laying on the hangers the compression is limited to bottom only but there is nothing helping with the stretch so the flex will pull hard on the 2 attachment points one on the duct the other at the boot. These are supposed to be taped and screwed but even at that the flex liner can unravel whether screwed or not.

Personally I would leave it. If you are concerned about energy lose then wrap some extra insulation around it at those points.

 

No reason that I can think of.
Is this new construction?

 

yes, new construction. Am I being too picky to ask them to spread out the hanger?

 

It's your home so you should get as you please.
However, if the rest of the installation is well done I wouldn't want to sour a good contractor.