In a multirotor, structural deformations & insufficient rigidity against a flight controller & reduce control on a good day, and set up constructive interference that manifests as potentially dangerous resonances on a bad day. To the human eye, though, they're very difficult to analyze because of the speed with which they occur.
An ideal technique for detecting fast cyclic deformations would be high speed video - unfortunately, those systems cost exorbitant amounts.
A novel means of shortcutting that cost is to run a cyclic strobe, which can be made to flash on and off very cheaply.
Described by Shane Colton:
One of the issues I've had with a quadrotor made from a single 0.063" printed circuit board has been vibration of the frame. I set up a Strobotac to flash at a frequency just faster or slower than the propeller. What you see is the aliased motion of the frame. For example, if the prop were spinning at 6000rpm (100Hz) and the strobe were set to 101Hz, you would see the frame vibration at 1Hz! This is shot with a regular video camera.
To counteract this vibration, I carefully balance the props. Also, the inertial sensors are mounted on foam tape and heavily filtered.