The heavy burdens were sufficient that the people of Limhi rebelled against them. Mormon’s statement that they could not deliver themselves because they were surrounded suggests that they were willing to abandon their lands and escape. While not mentioned at this point, it is clear that they were looking to return to Zarahemla.
At this point, however, they could see no way to escape. Therefore, they desired to obtain some independence through force of arms. That would break their oath. Perhaps surprisingly, they were willing to do so. That suggests that the conditions were indeed onerous.
They do attempt to fight, but they are driven back. The previous encounters had them victorious, but subtly involved in those victories is that they were defensive. This was an offensive military movement. It did not succeed.