They don't have the longevity that's required. Tiny cracks can form under the sealer, which can't bridge them, and water gets in. He recommended elastomeric pigmented coatings. Technical Director George Domedion pointed out that there are EIFS fake stuccos, which have the look of stucco with an acrylic topcoat. That has a pretty decent water repellency.
Real stucco is a masonry surface that is porous, which demands a topcoat to avoid absorbing water.
Both will potentially have cracking at some point. With an acrylic system, Domedion recommends using the original compound it was made with so it has the same consistency. That will generally move with the crack. Then you can just go over the top with a patching material, and finish coat after that. Cracks can also be caused by shrinkage of the mortar, due mainly to improper drying when constructing.