You're less likely to be run up by someone on a fixed (with brakes, as is the discussion here) than someone on a geared. Geared riders try semi-useful ways to slow without braking, like sitting up to catch air. It doesn't always work, which is why many runups happen. In contrast, fixed riders can control their speed more precisely, and less often have to touch the brakes to maintain their gap when the riders ahead fred out.
On the other hand, what's an advantage to the fixed rider can be a problem for the inattentive guy behind. Fixed riders have to concern themselves with runups a lot because they'll maintain their gap without transmitting the "fred alert" back the line - no sitting up, no coasting, no grabbing brakes, no swerving, no cussing - just calm, perfect gap control. Then the fixed rider gets blamed for the sins of the guys ahead.
I don't like mixed pacelines as a fixed rider. Never had a fixed rider in a line when I was riding my "weakling bike"

, but I'll assume I'd dislike it equally.