LuaJIT has been stable for several years, and has IMHO a low chance of making major breaking changes, because it includes thousands of lines of code in seven different assembly languages, plus the lead developer seems to be burned out with big changes and is just keeping the lights on (2.1 has been beta3 for a long time). Nonetheless it does have eyeballs on it. It's also easy and fast.

One issue is that it has diverged from Lua but still shares the same name. Mainstream Lua will probably continue to be a rapidly moving target.