Extremist parties will rise under PR.  A democracy means that everybody’s view should be valued equally. However much some people might dislike one party, other people still have the right to vote for them. If extremist parties do gain seats, then it shows the government needs to do more to address concerns. Moreover there are plenty of ways to avoid this problem in a proportional system, for example the German model of setting a lower bar that needs to be crossed before a party can have any representation in parliament. This would show that it is a party that can attract sufficient support to be worthy of representing those views in parliament.