I would disagree. Not only does the CPC have an opposition party, it has 7 opposition parties in the central Government, and hundreds of opposition parties in specific local elections, and hundreds of non-affiliated opposition. Operating in a coalition does not mean they are without an opposition, coalitions are not always going to agree on issues. It also has to tackle an entire House, where the CPC has 0 party members (because it isn't allowed any), and a Supreme Court, where the CPC also cannot influence decisions, and has the power to strike down CPC legislation and challenge the Government.
I would say this misconception that the WPK in DPRK, CPC in China, are completely unchallenged, is one of America's greatest propaganda successes. It is filled with so many subtle influences you wouldn't even notice unless you looked. For example, American media refers to the CPC as the CPP, the Chinese Communist Party, instead of their real name, the CPC, the Communist Party of China. This subtle language change that has become widely adopted implies that the CPP is the Chinese government, as in, the only party to exist in China.
Not only does the CPC constantly have to haggle with opposition and independents at elections to maintain its United Front, it also gives its large autonomous regions complete self control in much of their affairs. The Central Government has less power than most people would think.