It looks like someone used wood filler to fill in gaps and it didn't cure properly, now it's brittle. 

Some of that really early insulated wiring is kinda trash--the insulation gets brittle and disintegrates, which is (to put it mildly) not the best, but that's not what he's saying. 99% chance you either misunderstood or he is lying.The bathrooms have mold because they're damp.

That's also very useful info about the rodents--one more reason to take preventative measures.I was thinking of it from the perspective of how materials break down (especially with heat/cold cycles)--typically, rubber and plastic type materials break down faster in a car's engine bay than the metal materials, for instance...which makes me think pex would break down (become brittle) faster than copper....but, I'm no engineer, so who knows...

I am concerned about brittleness of the hardi product although that product is superior for rot and bugs.  

And a clay pipe is very brittle and can actually be damaged from snaking.

Copper wire conductors in NM sheathed cable or in rigid metal or PVC plastic conduit has been the norm since the mid-1970s, and there are currently no new innovations in the wiring materials themselves.Most houses I have issues are from the 1950s old rag wire with no ground the cloth is brittle from heating of the current over the years.Lots of flippers will cobble together a house, install new wire where they are forced too leave old rag wire to rot away.

Spend the money on a decent tray or they get brittle and wear out.

My understanding is that over time, it becomes very brittle (& then breaks, obviously!)

Keep in mind that it's easier to see this stuff on a typical 3-tab shingle than on an architectural shingle.A roof that just has a few torn tabs but is otherwise not super brittle and functioning properly?

The water will be flooding.And the walls, carpet, other surface features will have a smoke smell, and possible mold smell.Chimney may be cracked from the fire suppression and heat with water.Tile becomes brittle from the heat.Demo can be real expensive because roofing, flooring, walls, all can have asbestos.