Opinion on Category C boots for summer alpine climbing

As long you're not climbing north faces, C boots are very nice. I have been on La Sportiva Cubes for some years, now I'm actually going to switch to C boots again cause it's more comfortable and D shoes aren't really needed since the hardest I go is AD.

As always, depends on what you're climbing. I like C for rockier stuff with not that steep glacier approaches, and D for frontpointing weird stuff. Weather-wise both C and D can have similar amount of shielding. I have fancy high-dex D boots (Scarpa Phantom Tech) that are less warm than my ...highly Teutonic Cs (Meindl Litepeak Pro)

It mostly comes down to how well they work with crampons and whether you are climbing routes that require consequential front pointing in your crampons. If you are not going to be doing any consequential front pointing, softer boots are better. If you are, stiff soled boots are better.

Regarding Insulation, if you go with a boot that is less insulated than the La Sportiva Nepal or Scarpa Mount Blanc, you will either be restricted to moving faster and more continuously than the typical novice mountaineer or limited to warmer weather periods.

I climbed Mount Blanc (June) in Nepal Evos, and my feet were quite cold near the summit. Had I been in boots with less insulation, I would have had to turn around.

C is fine for most normal glacier routes that don't require excessively steep frontpointing.