Then you have been very lucky so far.

The Internal Oscillator is rarely on-frequency, it drifts with temperature and has the stability of a political manifesto at election time. The higher the baud rate, the greater the error margin. If you send or receive a short burst, the few bytes might be within spec, but thereafter you will drift out of specification and communication will be corrupted.

The rule is always to use a Resonator or Xtal when doing timing critical functions.