The problem with art is that people have been forging works forever. It's impossible to get a feel for how many fakes are out there because so much art is passed down within families and never makes it onto the market and therefore never seen by experts who can spot a forgery.
My old man claims the whole art world is in on the take. Picasso claimed he painted fakes all the time and Matisse famously said "I have painted 2000 pictures. 5000 of them are in the USA."
The predatory nature of the art world allows for fakes to more easily enter the market. For instance, my dad was selling his pieces for a couple grand each and the dealer he sold to was reselling them for upwards of 20x what was paid (per court documents). If it seems too good to be true, it didn't seem to matter. The dealer wasn't charged and is probably still quite happy to sell fakes for a huge profit, caveat emptor and all that.
Anyways, that's my observation from the sidelines of the art world. I prefer stable investments like Beanie Babies and Nascar collectable plates, not paintings.