Keystones shouldn't have any higher occurrence of leakage than any other RV at that price point. The method and products used for roofing are quite similar across the spectrum. You probably have heard or read a lot about Keystone because you are researching Keystone. If you searched equally amongst brands, I'm sure you would see their rate of "failure" is similar to other brands.
Keystone is an entry level trailer. Fit and finish aren't the best. You are looking at a lightweight trailer, so the trailer is going to be built lighter. That means fewer wall studs, likely the floor is lighter and joists are spaced further apart, the frame is likely lighter, cabinets are lighter, etc etc.
You won't be getting as solid a trailer (because its liteweight), but roof leaks are usually the result of poor or insufficient maintenance by the owner... not quality of build.