Sure, rising and falling edges are more demanding in terms of harmonics -- but most engineers understand that, because it is the example they were taught. Ramps live in the danger zone where they look sort of sinusoidal, "requiring few harmonics," while presenting a severe temptation to assume ideal linearity, which brings the requirement for those harmonics right back, especially at the sharp up/down transition points.
Ramps are the perfect trap for young players, in other words, and if instrument manufacturers aren't careful they will take the blame: some kid will claim their experiment was ruined because of the manufacturer's bad ramps when in reality the young player should have abandoned the concept of ideal ramp stimulus long ago, even if doing so required a bit of math for a different stimulus function or an extra reference measurement channel.