For anyone still on the fence or against CSSJB > SSJB Bursts, just look at how Vegeta was managing against SSJR Black.
While it's not arguable that Vegeta wasn't going for the kill, there is no reason to believe that Vegeta was holding back power. Black was still capable of blocking some of his blows, and it took a fair amount of time for Vegeta to really wear him down.
Merged Zamasu was ecstatic about how powerful he had become.
Ch. 23, pg. 15.5-7
Context: after Zamasu’s huge explosive wave
Zamasu: “Perfect… This is too perfect. This is amazing!! This is devastating power!! This is it! This is what I’ve wanted!! Look at this, filthy Lord of Lords!!! The time of you feeble Gods watching over everyone is now over!!”
Do you really think that the guy who vehemently hated mortals would be so excited about being equal to their level? I think not. And we know that Goku was able to match this level of strength using CSSJB.
Furthermore, the main difference between CSSJB and Bursts is that the complete form is sealing the power of Blue within the body. Using Bursts might make the transformation more efficient and easier to maintain in the long run, but power is still escaping when Blue is used. It can't be using all of Blue's power if the power isn't sealed within the body.