As discussed in https://bugs.webkit.org/show_bug.cgi?id=37984#c28 and below Nicolas and Behdad suggest to go the Firefox way, optimize the complex font path using caching for shaping results of TextRun and phase out the simple font path altogether.
Their argument is, the simple path + the heuristics to change to complex path are too fragile. In too many cases the simple path assumption fails.
I am creating a meta bug to discuss and track this.
BTW anyway wanting to keep the simple path should study African scripts, as they are doing mightily weird stuff on a latin base.
CC'ing Dan. I created this bug mainly to start the discussion - and follow up on the suggestions Behdad and Nicolas brought up in the previous thread.
Dan, you created for example bug 100050 - which probably touches on similar issues - I am not sure this one and similar issues can be solved with workarounds applied to the simple path.