Leaving the specific politics aside... if party A routinely defects and party B routinely cooperates then the system is no more effective than party A operating on its own, and quite possibly less so, and this is reasonably stable unless party A is incompetent enough that it can't even maintain power in the absence of opposition. Whereas if parties A and B both routinely defect the system is far less likely to be stable.

Whether that's better or worse seems to depend a lot on what the outcome of that instability is... that is, if both parties start defecting and the existing system is no longer viable, one important question is: what replaces it, and how soon?