In Line Abreast all airplanes fly wingtip to wingtip, with only about one meter in between each wingtip. This formation may look quite simple but it is notoriously difficult to fly, especially when flying turns. The outmost aircraft needs to fly a lot faster than the leader and the inmost aircraft needs to fly a lot slower.
This poses a challenge, because the outmost airplane may lack the power to keep up. The inmost aircraft may fly only a few knots above its stall speed. Therefore, the leader needs to monitor his speed very carefully as not to fly too fast or too slow.
Finally, from the perspective of the audience, small differences in the distances between the wingtips are easy to see. The pilots have to not only monitor the leader, but also the planes next to them (when they are not flying directly next to the leader) to maintain the correct altitude, position, and distance to one another.