"Checks and balances" mean nothing unless you specify who's checking who. System where the military keeps a check on the president, or the king keeps a check on parliament, aren't really much better than the "checker" being in charge all the time. And systems based on mutual checking tend to just reel from one constitutional crisis to another.

The quality of implementation definitely matters, agreed. However, the Soviet states by and large didn't even have a shitty form in place, the party leader was an 'elected' dictator.