After years of questioning it , I am starting to understand the reason why people are crazy about OpenUI. For decades, the crude activex based frontend was a big pain area in Siebel. Customers were always using an older, outdated version of Internet Explorer for Siebel CRM, because the latest version never supported it. There was also that crazy 32bit, 64bit IE version problem.
So when the move to a open standards based frontend was announced, it was welcomed with open arms. No more IE. Any browser, any device. But then started the crazy ideas, crazy configurations, outrageous themes. People started implementing all their business processes purely in OpenUI ! I have stayed away fro this kind of work, for a long time.
But OpenUI works poses its own challenges. Unlike tools, there is no object locking system, so better have some kind of CVS in place if you have multiple people working on OpeUI files. Also, test in every browser. Do not assume that IE or Edge will honour the code the same way Firefox does. Test your code specially with the Show More button on the applet (tip), because Show More has lots of issues in OpenUI.
And finally, Siebel still provides the best architecture for a CRM system. Try to implement whatever you can OOB, and OpenUI should be the last option. Unless you want to get into maintenance hell.