Aside from being incredibly low on bugs (often, there are no known bugs at all, although complaints are likely to be treated as feature requests rather than bug

Aside from being incredibly low on bugs (often, there are no known bugs at all, although complaints are likely to be treated as feature requests rather than bugs), PUC Lua is also far smaller, and vastly easy to hack on. There are various patches available for it. On the other hand, the guts of LuaJIT are difficult to say the least. Most opportunity to eek out a little more speed by making the code twice has dense has been taken. However I found that adding custom library routines to LuaJIT (for operations like concatenating tables) was actually pretty easy as long as you don't want to be able to JIT them (since library routines call into an internal API which is well shielded, and there's still a lot of PUC Lua code in LuaJIT). Also, LuaJIT has other extensions aside from FFI; see the website.