If you're looking at Bolk's site, there certainly is some nice stuff in wonderful condition. The safety issue probably isn't as important as the investment issue. On safety you would need to remember that most of those guns were designed to be shot with loads that we would consider very light by today's standards and that shot with appropriate loads and always being careful the pieces should be safe. The investment issue is another matter. Those guns that were built in the first half of the 1700's are in great condition because they were basically never used. Most have bright finished metal, etc. The very first day you have them out on a humid or rainy day, the condition is going to degrade no matter how careful you are. The first main or frizzen spring you break or sear nose that breaks off you will have significantly devalued the lock. The time you fall down and break or significantly scratch the stock you will have lost lots of value. Those 18th century gunsmiths aren't making any more pieces, can we take the chance of losing or significantly damaging one that has survived?
I shoot old English and American stuff, but only pieces that are in good condition but which show period repairs, lots of wear, etc. Things that a scratch or two won't make much difference on or replacing parts is only adding to the previously replaced parts. Would I buy a Bongarde rifle or fowler, absolutely if I could afford it. Would I take it out and shoot it? Maybe once on a nice dry sunny day. Would I take it out hunting? Never.
Tom