The WebDriver protocol is standardized by the World Wide Web Consortium which makes it easy for third-parties to get involved and contribute to Selenium. WebDriver has been the foundation of many additional automation projects besides core selenium and has become the de-facto standard for UI autom...
Low activity on the main github repository shows that the community around it is shrinking and there's a decrease in developer activity. Usually this implies that the tool is in a not very good & updated state. The issue tracker also doesn't evoke confidence.
Watir introduces its own API which differs from the underlying selenium-webdriver. That means jumping to any other library which exposes the selenium-webdriver API such as Appium becomes harder as paradigms can't be transferred between the two.
It's very easy to go between UFT and LeanFT. Whoever is used with UFT can easily become very productive with LeanFT too, but with the added benefit of moving the testing closer to the development cycle.