Also a general problem is that HotSpot does not tolerate memory corruption, but for memory-unsafe languages you'd quite like to have an as-reasonable-as-possibl

Also a general problem is that HotSpot does not tolerate memory corruption, but for memory-unsafe languages you'd quite like to have an as-reasonable-as-possible debugging environment even if 60% of the heap was zeroed by bad code, which throws a pretty large wrench in being able to complete a "just deoptimize everything" as the first step to debugging.