One of the disadvantage of the Oppo R11 camera is that it's telephoto camera, the one with the 20MP, features a slower f/2.6 lens. The second normal 16MP camera has a f/1.7 aperture lens. The different is 1.226 stops between F1.7 and F2.6 considering the same shutter speed. It probably would be less of a problem if the camera's sensor had large pixels, but with 1-micron pixels, we can expect a mediocre low-light performance from this specific camera.
The 20MP are good for portrait shots where you can capture portraits with great amount details. I assume that this is one reason OPPO opted for such a camera. This means that you should consider which camera you shoot with when shooting in poor lighting conditions. I'm sure many Oppo R11 users will just switch to the telephoto lens to have zoom in closer on the subject, maybe not knowing that this move has an effect on the image quality. It's not the first phone that has the same "issue". The Xiaomi Mi 6 has the same kind of setup with a slower aperture for its telephoto lens. The Mi6 has two camera, one with a 27mm f/1.8 (equivalent) lens and a second 52mm f/2.6 telephoto lens. It's telephoto lens also has a sensor with small 1.0-micron pixel size, whether the wide-angle lens has a sensor with large 1.25-micron size pixels.
This is something to keep in mind when shooting in dark scenes where shooting with the normal lens will yield you better results. By saying better I mean photos with better exposure, better color fidelity and better dynamic range, because those shots are taken at lower ISO. The photos will also have less image noise because they are shot at lower ISO sensitivity.
Of course it would be great if the two lenses had a fast f/1.7 aperture or the second lens had a faster (below f/2.0) aperture lens, but this will increase the cost of the camera setup and up the phone's price. It's not a cheap phone regardless, so with better optics it would have been even more expensive. So compromises had to be made and this is what we get. F2.6 isn't actually that "slow" in lens terms, it's a fast lens, but it's slower compared to the F1.7 lens. In mobile phone terms it's considered as slow because of the small sensor and small pixel size that most phone cameras have. The lens should be fast to compensate for that.
It the Oppo R11 F2.6 a big issue? of course not, but it can compromise your low-light shots and it's just important to be aware of that when shooting in those type of shooting conditions. The great thing about digital cameras in mobile phones is that you get to see how the image looks like before tapping on the screen to capture the shot and when switching between the two cameras. The one thing that is sometimes harder to see is the noise, but you can see which ISO the camera is shooting at, so you can prioritize the cameras accordingly.
I see this as a disadvantage obviously, but it's not a big deal for most people. You can always switch back to the first camera that has a faster aperture and get good low-light pictures out of it. It will be interesting to see some camera sample images taken with both cameras of the Oppo R11 just to see the implication in real-life use. I'm sure some mobile phone reviewer out there will do it in the near future.