Canada

Just like the govt guarantees the pensions if the fund fails, it can also take excess surpluses. That seems totally reasonable?

The government doesn't guarantee the pension if the fund fails, they manage it so it doesn't fail.

EDIT TL;DR That means they're not financially liable to top up the pension, it means they get to decide how the pension returns to a healthy state, potentially by rewriting the contribution rules.

Most of the unions assume this means the next step is the government will stop funding their portion of the contributions instead of sharing the savings with employees. The step after that is if (really, when) the fund becomes unhealthy, the government gets to unilaterally decide how to fix the fund. Sure, they could top up the fund completely out of public revenues, but they could require employees to help top up the fund.

The reason why the unions think this will happen is because it has happened before. Any employee under the PSPP who's been employed for greater than ~15 years (I can't remember the cut off date) makes 35% of the contributions to their pension, with the government making the other 65%. Anyone who's been with the public sector under that cut-off pays 50%. That's because we started this same cycle back in the early 2000's with the government taking the surplus, but not putting the money back when investment returns were low. AFAIK, a similar cycle has happened at least once before that.