There is a lot of conflicting information about this.  In addition, there are no studies that can conclusively prove the injecting Bellafill is better with a cannula than a needle or vice versa.  One of the complaints about Bellafill is that it can sometimes make lumps.  This is largely due to two things.  First, the needle that is used to inject Bellafill is considerably larger (26 gauge) than the needles that are used to inject most other fillers (27 to 29 gauge).  This can result in bleeding which can cause lumps or small areas of clotted blood under the skin.  Second, blood mixing with the collagen in Bel​lafill can also lead to early lumps or scar.  These lumps flatten and settle in time, but it can take up to 3 months for that.
The main reason for using a cannula is that fact that with a cannula, the risk of injecting filler into a blood vessel and causing a catastrophic vascular injection injury is all but eliminated.
Aside from these risk mitigation factors, a cannula, because it causes less injury at the time of the injection creates a more precise result.  There is the added benefit of having a less swelling and bruising potential with a cannula.
In the end, the results can be the same but given the benefits, I prefer a cannula because of the very precise placement and thus improved results that can be achieved when using a cannula for Bellafill injections
Here is a video about filler safety for you.