First of all industrial robots drive trains with basically no play/backlash, plus the whole construction is very rigid. AFAIK they all use harmonic drive gearing, which is extremely expensive. Most DIY'ers are not willing to spend that kind of money, and that is one reason why they have jerky movements. 

With regards to Universal Robots. They use motors from Kollmorgen, with integrated harmonic drive gearing. 

In the Universal Robots FAQ ([link][1]) I found the following. 

 - Motors: Permanent Magnet AC Synchronous Drive motors are used in both CB3 and e-Series robots. 
 - Motor drivers: Integrated Motor Drives, Space Vector (Field Oriented) Control @25Khzhttps://en.wikipedia.org/wiki/Vector_control_(motor)
 - Servo controller: Integrated Servo Controllers in each joint, based on a dual encoder system

  [1]: https://www.universal-robots.com/how-tos-and-faqs/faq/ur-faq/what-type-of-17271/