I have been using straight for a while now and I think it is great! The ability to lazy load everything by default does a lot to make Emacs snappier (or at the very least, faster to boot). Being able to pull packages directly from git (be it local or a forge) makes package development a lot easier. raxod has a lot of really sleek, modern emacs packages that I would encourage everyone to check out, selectrum[0] and ctrf[1] in particular are really great as well.
You can get autocomplete for Java methods in Cider (Emacs) and probably Cursive (Intellij) afaik. Type checking in intro code can be a bit difficult, but REPL/live development really helps. You are going to catch mismatched type at first call, so you can usually get there by reading the Javadoc, but it does take some practice. Most Clojure code also works with Java through the smallest interface possible at the edge of your system, so it is very rare to have to deal with those issues in your "main" code.
As much as I admire Processing for making it easy to draw pixels on the screen, something that seems to get more difficult every year, I do find it very difficult to use as a teaching tool. Going from Hello World to something more intricate is a bumpy ride with very few affordances, the feed through from drawing a rect to interfacing with I/O is, while obviously a difficult problem, non-existent. "Creative coding" also requires both a high degree of mathematical and artistic capability in order to be something other than a soup of primary color on the screen, something beginning programmer (esp. "non-technical" ones) tend to struggle with.
That said, I still really admire what they did and how extremely influential it has been. Just a small thing like how the the Processing IDE was forked to create the Arduino IDE attests to how Processing continues to reverberate throughout history.
preview.app has been around since NextSTEP, which used display postscript for its windowing engine. Having a first-class pdf reader falls out pretty naturally.