air source have the reputation for freezing and failing to work in cold/near frosty weather. I imagine they are cheaper to install than ground or water.
There are a few committed self-publicists on the net who are opposed to CWI, one of them is associated with a newspaper. It makes me think of the people who are opposed to vaccination.
In the rare event that it is incorrectly installed in a wall that is wet (this is usually on cracked walls, or with defective pointing or render allowing water penetration) the installer, who should have observed the fault during survey, and refused to install CWI until it was rectified, can be forced to remove it at own expense. AFAIK this is very rare, but I would always choose a large company with a reputation to protect. BG usually offer free insulation, even to people who are not its customer. The other large utility companies are I am sure equally trustworthy. They treat millions of houses at low cost, and I can't see it would be profitable if they often had to go back and correct mistakes.
If you have a house that already suffers from condensation, insulating the walls will make them warmer, so more of the water will conspicuously run down the windows instead of soaking into the plaster and drying out when you open the windows.
If the brickwork, render or pointing is defective, or the gutters or downpipes are spilling water down the wall, it should be repaired anyway, whether or not you go for CWI.
The other cause can be a badly-built wall with accumulations of mortar dropped down the cavity by the bricklayers. It is possible to identify these by infra-red or miniature camera inspection, though I have never seen it done. Mortar can be raked out by removing a few bricks at a time for access. I have done this by hand, and it is very tedious. I presume it would cost at least some hundreds of pounds.
Get the surveyor to inspect the walls for possible damp before going ahead.
IME CWI is an excellent investment, it improves comfort and cuts energy bills.