Generally I'd say public sector unions (especially in essential services) are of very questionable benefit and need limits, but robust private sector unions are much more obviously beneficial to society.

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In the private sector the incentives are mostly aligned for producing reasonable deals, because both sides rely on the business being healthy and making a profit and the jobs fundamentally rely on that.

In the public sector they aren't aligned. The politician is most incentivized to avoid immediate political turmoil. Voters are not market analysts who recognize and have a problem with deals that produce massive costs in the long-run (ex: exceptionally young or exceptionally generous retirement). The union is often aware it can extort the public with the threat of causing chaos. Government can raise taxes/take on heavier debt, which further weakens it's negotiating position - in all but the most extreme cases it won't be going into bankruptcy or ceasing to exist, taxpayers in 30 years will just be on the hook for paying a bad deal made by a previous generation.