Proportional Representation might lead to coalition government. A coalition government is the government formed when no one political party has achieved a clear majority. As a result, different political parties must work together to form a temporary coalition in order to rule.
Under first-past-the-post, there is usually a political party with a clear majority. However, due to the nature of Proportional Representation, it is more likely that coalition governments would be needed. Many have written about how the likelihood of coalition governments is raised 'significantly'
by Proportional Representation.
Coalition governments have been very rare in the UK's political history. The Liberal Democrat and Conservative coalition government of 2010-2015 was the first since the War Ministry during the Second World War. 
In contrast, in countries with a history of deploying a form of Proportional Representation in their voting system, coalitions have been the norm rather than the exception. For example, in Germany, It is usually the CDU/CSU who rule along with another party, and indeed currently it is an alliance between the CDU/ CSU and the SPD.
Detractors of Proportional Representation argue that a coalition government is a less effective form of government, due to the competing interests and ideologies at play. Any given political party already contains internal squabbles and differences in opinion, and this is only magnified when there are two separate political parties with separate ruling visions.
It can also take too long to form a coalition government. In 2010, it took Belgium 541 days to form a government while it took Germany, and over 160 days to form a government after its September 2017 election.
This uncertainty and indecisiveness can detract from the bigger issues.