We’ve been on seven river cruises so far (with Viking and three other river cruise lines) and plan to go on several more – we absolutely love them! Yes, they cost more up front, but so much is included it really is a great value (airfare can be really expensive though).
Regarding the river being dried up: it’s impossible to perfectly predict the weather, so it’s possible, but it certainly isn’t normal or there would be no such thing as river cruising. Recently there’s been heavy rain in Europe causing the rivers to rise and some ships couldn’t fit under some of the bridges. The river cruise lines have lots of plans ready in case a river is too high or too low. We’ve only been unable to reach a port once but all of the excursions went as planned, we just had an extra 30 minutes or so on the motor coach that day. We’ve talked to others who had to spend a night or two in a luxury hotel (with all meals included) before meeting the ship upriver. We’ve even heard of people having to swap ships mid cruise; one ship’s going north, the other south, but there was a bridge in the middle they couldn’t get under. The ships had the identical layout so the cruise line packed everyone up and they traded ships – folks said it went flawlessly!
Bottom line: sure, the river could be low or high, and it could impact your itinerary just like bad weather can make a big ship change plans, but the cruise line is going to do everything they can to make sure you have a wonderful trip. And sometimes those surprises end up making the best memories!