To make Unique pens from wood with gnarly knots takes a lot more time. But the payoff is a pen with lots more unique character.
The knots in wood are often harder than the surrounding wood and have a tendency to be brittle and happy to break apart at the slightest touch. A steady approach with freshly sharpen chisels is the only way to tackling these very difficult to handle woods. Which is why they are often left to one side by other pen makers.