As a long time Kettle user (probably close to 10 years) I must warn potential users that the learning curve is steep and that (as any large body of code) it contains code that sometimes can run unpredictably. I got good at diagnosing user induced bugs in PDI transformation via reading the stacktrace but it is not to everyone liking.

To me, a very strong regression is the "new" UI which switched from meaningful icons to a blue & white scheme that makes reading/discovering new transformation a real pain: all is a blur of blue without the past color cues that you learned in the past ("ok, this is the icon for a merge from a source file & a database sent to an ES cluster" became "some stuff is read from blue sources and sent to some blue output")

I recently learned about the capability to run transformation into a spark cluster that replace the original engine by a new spark implementation, bring obvious compute optimization for large enough dataset but I don't have enough experience with it to speak of it positively or negatively.