What is the risk of hidden physical defects?

[Seth C.]

It seems easier for a professional inspector to find the problems with an apartment building than a mechanic inspecting a car, since so many vehicle systems are sealed. At the same time, there is some risk of missing issues in roofs, HVAC, plumbing, electrical, even appliances, etc. 

Are there any experienced investors out there who can help quantify these risks and/or point out what one might focus on? How big of an issue is this? I have a decent amount of experience in construction, but not enough to answer this to my satisfaction.

Thanks!