From my understanding, and stated within QJB's post, the disadvantage of rearwards sweep is that you end up with a spanwise flow. This thickens the boundary layer towards the tip. The result is earlier separation at the tips, and therefore earlier stall. The use of fences on swept wings was there to prevent this flow and help relieve this condition.
The consequences of a tip stall on a swept wing design can be more dramatic in terms of pitching moment. This means that not only are swept wings more susceptible, but also the need to prevent it is greater.