here's my experience, found a rhody, dug it up with a ton of roots, chopped it like crazy till there was nothing left but two main trunks, no leaves or anything, stuck it in a pot that it barely fit in, a couple days later, attacked it and took off a bunch of the roots, potted it up again so it would have more space, and it is doing great

i did all my chopping while it was flowering, so as far as i have learnt it can be cut back hard and root pruned hard and be fine