I'm in a union and I still criticise unions for this. You can't make it harder to fire people unfairly without also making it harder to fire them when they're actively harming the business and/or their co-workers.
I've known of two engineers whose managers told me they would have fired them, but higher management wouldn't let them initiate the PIP process. The one case where I worked at the company was pretty bad. We had to shuffle the individual off onto makework jobs where they couldn't do much harm.
I don't think it's fair to blame this entirely on unions when it's the result of big businesses being too scared to follow a process that was given to them by the government. Unions mostly fight real unfair dismissals and only play a minor role in creating a chilling effect. Still, in practice it's hard to fire someone.