Null-terminated strings try to save space by encoding the string length by convention. This can fail due to off-by-one errors, mistakenly allocating a fixed buffer, strings that have an unexpected null in them, and more.

Those can lead to all kinds of nasty problems like buffer overflows, which allow someone who can craft an input to write arbitrary stuff into memory.

God only knows how many security vulnerabilities and performance problems could be traced back to null terminated strings.