Roller reefing on a headsail is absolutely fine if all you will ever do is puddle around in modest weather; or go with a full reef and start the iron spinnaker
But in winds of serious sorts, roller reefing of the headsail increases issues.
Think about it, it's just physics. When you genny is set at full and your main at full, the vasrious centres of effort (the sails) compared to the cente of lateral resistence are in harmony...At least they should be.
Moreover, the wind on a full-set headsail passes by with minimal obstruction.
Now roller reef in your headsail. Apart from lessening the sail size, what does it do? First it creates a huge source of leading-edge eddie. Second it lifts the centre of effort by the degree of furl.
Do you all know how to calc the centre of effort of a sail? Sure you do. Tripex three lines
from head, clew, and tack, to the opposite centre, (head to centre of foot. Luff to centre of leech, clew to centre of luff) and the trisect is the centre of effort.
See how roller reefing lifts it ever upward and creates and ever-increasing horizontal load on your boat?
Slab-reefing of a headsail creates none of these issues. As a slab comes down, so does the centre of effort, and the luff is still nice and clean.
Slab-reefing of a headsail is way-less arduous than roller reefing and presents none of the issues presented by roller-reefing. But it's essential only when one is playing in winds exceeding 25 knots.
I hope that helps.