Various bits of what is known about busybox shells, in no particular order.
+2008-02-14
+ash: does not restore tty pgrp if killed by HUP. Symptom: Midnight Commander
+is backgrounded if you started ash under it, and then killed it with HUP.
+
+2007-11-23
+hush: fixed bogus glob handling; fixed exec <"$1"; added test and echo builtins
+
2007-06-13
hush: exec <"$1" doesn't do parameter subst