One of the slides in Blow's talk about why he was starting work on Jai said, "If we spend a lot of time wading through high-friction environments, we had better be sure that this is a net win. Empirically, it looks to me like [that] does not usually pay off. These methods spend more time preventing potential bugs than it would have taken to fix the actual bugs that happen."
I think that's an overall good summary of the crowd's attitude. They think that mainstream programming environments err too far in the direction of keeping your software from being buggy, charging programmers a heavy cost for it. Undoubtedly for videogames they are correct.
Jai in particular does support array bounds checking, but you can turn it on or off as a compilation option: https://jai.community/t/metaprogramming-build-options/151
And stability is important, but not critical - and the main way they want to achieve it is that errors should be very obvious so that they can be caught easily in manual testing. So C++ style UB is not great, since you may not always catch it, but crashing on reading a null pointer is great, since you'll easily see it during testing. Also, performance concerns trump correctness - paying a performance cost to get some safety (e.g. using array bounds access enforcement) is lazy design, why would you write out of bounds accesses in the first place?