Downside is you don't know if a rabbit is not drinking due to illness. With a bottle you can measure it, and if you need to medicate the water for a rabbit you need a bottle. If it's hooked up to a garden hose you cannot give apple cider vinegar, if you choose to one day or if you already do. If it's gravity fed from a bucket then you can give the acv. Also hard to clean water lines and know how dirty they are, easier to tell with a bottle.
But in warmer climates you have to fill bottles more than once a day. If you don't have a lot of time to spend on doing chores, then the watering system could be real handy... If i had a gravity fed one, i would do that for summer months (as water lines would freeze here). I don't have hydro, so i wouldn't be able to have one hooked up to a garden hose.
If it's a good price i would try it...but you may be able to make your own cheaper. Price things out.