Breathable is another word for 'controlled leakage'. Use the wrong detergent, expose it to enough salt/dirt/crud, and it will leak. Breathable membranes are more delicate than textiles that don't try to split the difference. There is a reason rough use tarps and ground cloths don't use breathable fabrics.

Breathable fabrics are an acceptable trade-off for a rain suit, or bivy where ventilation is difficult and failure means your own sweat defeats your insulation. Add that to the simple fact that you are going to pay a premium, and have to take more time and care in maintenance, and ponchos made of breathable materials just don't make sense. Stick to a poncho that doesn't have leakage built in and use it hard with confidence that it can take it. When it wears out buy, or make, another.