C is designed to allow programmers to insert or remove checks as performance tuning. Compiler UB "optimizations" that remove that ability from the programmer make the language unusable.
Not knowing what is UB in C is equivalent to not knowing a core part of the C language. In that situation, yes, C would seem unusable for people who have superficial knowledge of it.
Nobody knows UB rules. The WG14 discussions are full of confusion.
Send Linux Torvalds a note and tell him he has only superficial knowledge of C.