"effecting something else" (i.e. escaping the sandbox) is the core issue. JavaScript (and WASM) engines have to be designed to defend against the user running outright malicious scripts without those scripts being able to gain access to the rest of the browser or the host system. By comparison, potentially exploitable but non-malicious, messy code is basically a non-issue. Any attacker that found a bug in a sandboxed XSLT polyfil that allowed them to escape the sandbox or do anything else malicious would be able to just ship the same code to the browser themselves to achieve the same effect.