Slide-outs for the most part are no longer a liability. Any opening into the box has the potential for leaking be it a slideout,skylight or window. Nevertheless slideouts are a great way to get more space. They have been perfected and allow for a variety of floorplans.
Personally I would not buy a unit with out a slide. My current unit has 5.
Be sure to consider how accessible the unit is with the slide closed. My unit is enormous when the slides are out and fairly accessible when they are in, although I do happen to open a slide to access the fridge, which could be a deal breaker for someone. I consider it a issue I can live with to enjoy the added space.
The trend has been to add more slides vs. building units without slides.