Nothing's wrong with it; it just doesn't go far enough.

It doesn't define a standard module system or macros (beyond syntax-rules), exceptions, or record types. Most R5RS implementations offer those, but they're all different and incompatible. The standard also doesn't help with boring but necessary stuff like file encodings, character sets, etc.

So instead of writing a (nontrivial) program for R5RS Scheme, you have to write it for MIT Scheme or Racket or Chicken or Chibi or ...