What's up with the $20 bill being sought after by counterfeiters the most when its face value is shit in comparison to $100? Stores being scammed with fake cash that a pen is needed to distinguish between genuine and fake, its a problem they encounter quite a lot. US Dollars are printed with a single layer consisting of linen and cotton, which is easy for counterfeiters to exploit (like bleaching a $5 bill imprinting $100).
That's something you don't hear about as much with Swiss Francs as they have top notch security features (they're printed vertically at different sizes). Every denomination (from 10-1000) features a globe with security ink showing dynamic color changes and a shifting arc when tilted. The bills are printed using an innovative three-layer composite material known as Durasafe which involves 8 complex printing processes.
Unlike many countries they've never demonetised any notes, even silver certificates from the 1800s are still legal tender (although genuine ones have collector value, so you'd be an idiot to spend them). So even if they introduce security features people can just forge the old version.