cyrnel

Thermal contraction and expansion cause fatigue in materials. Shouldn't be a big deal if the computer has cooled but if a component is hot before you freeze it you could destroy it completely. The quickest way to destruction is to freeze just part of a larger, hot, component.

A charge can be created by walking across the floor, rubbing your nylons, etc. Moving air creates a charge. Quickly moving air does it quickly.

Vacuums aren't always dangerous but they can be, and vacuums aren't just dangerous because of ESD. Vacuums:

-can be unwieldy. Easy to knock components & cables loose. (the most common and immediate vacuum hazard I've seen)
-can conduct charges between the hozzle and motor, and are usually ungrounded.
-create their own charge through air movement, and can hold it until you touch a conductor.

The ESD problem comes from a combination of circumstances not unique to vacuums. Damage from ESD implies a charge, possibly generated over a long period, then a transfer of that charge to vulnerable components. Just about everything in your computer is either vulnerable or connected to something vulnerable.

There are ways to minimize problems. Keeping everything at the same potential by touching the case and vacuum hose at all times. Not shuffling your feet. Doing your work away from carpet, ad infinitum. (google ESD prevention)

The "don't vacuum your semiconductors" rule comes from industry and military experience. Through tool choices and habits you can be lucky for years but in general it's not the best idea. It's definitely not smart to recommend to someone who might use their 15HP shopvac to lift dust & those pesky parallel traces.

BTW, there are vacuums made for electronics cleaning work that have a nozzle path to ground. Use that in your disclaimers to be safe.