1. It stains plastic. Notice my Kaweco Sport demonstrator after I thoroughly washed it. I had to use copious amounts of rubbing alcohol and q-tips to get it clean again.
2. It smears on good/glossy paper (such as Clairefontaine 90g) even after it has had a week or more to dry. I used the Kaweco's broad nib so you might notice different behavior with a thinner nib.
In conclusion, caveat emptor.
