Yeah I'm not sure how it works when it comes to humidity, but I'd bet you're right that humidity will gather there as well, and that one would do well in factoring this in when building. I'd say the choice of your wall material is important. Maybe you could leave a slit of about 10 inches of free airspace above the trellis (and not let it grow all the way to the roof) to leave room for the humid air to vent out when it warms with heat from the building.
When it comes to repairs- yes repairs would be more difficult, but then again damages might not be as common since you essentially have a regenerating, living "hull" around the building. So if there's ever a storm, the plants take the beating, and not the house. And then it regrows.
The version that seems to be most popular is a stainless wire trellis arranged as a netting on the walls on which the plants grow, that keeps the roots away from the actual wall which would otherwise cause damage. I know that some people go the extra mile and cover their walls with hard plastic or aluminium sheets- this is mostly when using other plants than ivy, that may have more aggressive root development.
Sorry this is borderline OT, but I think you could do the same thing with willow. And there's the added benefit that willow grows extremely fast.
Check out their site. Sorry, it's in swedish, but I'd gladly translate if it's not too off topic. http://www.vegtech.se/sv/veg-tech-bygg/products/grona-vajern---vajersystem/uid-54/categoryinformation.aspx