It's not just the profile size, it's also the added weight of the wheels. Extra mass there means more is transferred from the already skinny sidewalls into the suspension and in turn into the chassis. You'll also accelerate slower and increase braking distance.
For some the looks are worth it (and I totally get that 19s look better than 18s). For some the knock on effects aren't worth it. Your call, but in answer to your question yes the ride will be noticeably harsher, but I can't say whether it'll be too much or not. As someone who has suffered with back issues previously (seriously herniated disc L5/S) I do understand where the question is coming from, and personally if it were me I wouldn't risk anything making my back worse.
But then I've no idea how bad your condition is!