How limiting is the free plan really? Can I shoot an actual wedding on it?

Honestly, no — not a real one. The free plan gives you 1 GB of storage and exactly one album. A single wedding from a professional camera will blow through 1 GB before you've uploaded the ceremony, let alone the reception. Where the free plan shines is as a try-before-you-buy sandbox: you get every feature switched on, so you can create a test album, upload some photos, run the face recognition, and see exactly what your guests would experience. Treat it as the demo, not the delivery vehicle.