The only problem with Roblox is its build settings. I would rather have a built version than a version hich has to be downloaded. Core and Roblox are similar in terms of being an engine but core has actual build settings and roblox sadly dosen't. Like do you expect the judges to download a platform just for the jam . If you can do some techinical wizardry and make a game in roblox have a compatible version for windows, linux or mac, go for it. Also the 600 lines of code thing is a really really bad argument. In that case YandereDev would be GameDev's bill gates