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Having used telescopes with a number of different substrates, I'd have to agree that just because a mirror is made from ZeroDur or Astro-Sitall that is by definition going to be a good mirror. In fact, I had to return a very expensive Sitall mirror to Star Optics for re-figuring due to a turned edge.
But the elephant in the room that Royce and Seronik don't mention is focus and figure stability. Provided the substrate is homogeneous and isotropic, bulk temperature changes only cause a focal shift. Temperature gradients will however distort the figure, and are harder to control as the mirror size increases.
Optics with other than a zero CTE can and do shift their focus as the ambient temperature ramps up and down.
If you are a planetary or visual observer focus shift matters not. But if you intend on taking 30-minute subs over several hours ....pain and frustration come to mind.
The two instruments I've owned with Astro-Sitall optics showed not even the slightest focus shift, in fact on my 36cm RCOS, focus was stable for weeks at time.
Figure changes are far more problematical. A thermal gradient across a zero CTE mirror simply makes no difference to its figure. That cannot be said for most other materials.
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