Earlier quoted context omitted.
What kinds of security issues? I really can't think of any besides some issues with too many threads spawned (that can be trivially blocked).
Could be a new source of undefined behavior and memory exploits, if threaded code is allowed to access collections that allocate memory without taking all the locks they should?
The VM should be sandboxing the pages from each other, so those are potential security threats against targets on the same pages only. Not something to be too concerned about.