The engineering and potential problems of fiberglass hulls are far too numerous and complex to summarize briefly. Suffice to say, the possible problems can be showstoppers and potentially make a boat virtually worthless.
The marine surveyor David Pasco has written extensively on the topic. Some people consider him somewhat controversial (usually, it seems, those who build boats with balsa-cored bottoms). There is a lot of good information on his website, and I also found his books to be very worthwhile:
Yacht Survey Online: David Pascoe, Marine Surveyor
Another thought, if you plan on looking at a lot of boats, you might also want to buy yourself a moisture meter, and pay a surveyor to teach you how to use it. The Electrophysics CT-33 is the same unit most surveyors use. I bought one and used it on over 100 boats in my 3+ year search. I wouldn't trust myself to "survey" a boat, but you can save a lot of money by ruling out boats with major moisture penetration problems (invest $175 in a moisture meter + $100-$200 to learn how to use it = much less than the cost of even a single survey). In my case, it saved me from paying for many surveys since I was able to rule them out myself.