Are all vintage bikes difficult to work on?
No.  Older bicycles are no harder to work on than modern bicycles provided you have specialized knowledge regarding older standards, possibly specialized tools and the ability to obtain parts designed for older standards.

Generally a bicycle built now will likely conform to a set of standards that are common and in place now.  If you bought a bike today, and purchased all the tools necessary to work on it today, you'd likely find that new bikes five years from now would have a lot of overlap, but some standards would be new/different and a few new tools may be required.  In addition, all the parts necessary to rebuilt said bike would be available.  Five years from now you would have a lower percentage chance of finding all the parts, but it would still be very high.  Additionally some of the newer mechanics may not be familiar with your "older" setup, but there would be many around that were very familiar with it.

Drag this out for 30+ years and you see where it is headed.  A "vintage" (a term badly stolen from the wine industry and used incorrectly) bike will have mostly standards that are no longer used, require some tools that may no longer be common, and may or may not commonly have parts available.  Additionally, 30 year veteran bike mechanics are a rarity, so someone with all the knowledge of said bike is going to be hard to find as well.  It simply isn't worth most mechanic's time to try and figure out a bike from sometime around when they were born or earlier (especially when they are paid by the job).  

People often buy inexpensive (or overpriced) "vintage" bicycles thinking that it should be cheap to fix up an old bike.  The truth is that it requires specialized knowledge, parts and sometimes tools.  It should be similar to what you would expect buying a 1978 Ford LTD that is rotting out and not currently running.