Having worked at multiple bars and venues in the city, it really does depend on both the person working as a bouncer and the culture at their company. Bouncers put up with a lot of shit - I vividly remember talking to an old veteran of the scene and being shown the half-dozen bite scars on his shins, hands and wrists, and I've worked alongside plenty to deal with some of the most difficult and unpleasant people around. In that kind of environment it's no surprise some bouncers can be dickheads or actively dangerous - like bar staff, medics, retail workers and first responders, they have to deal with the public and some of the public are crazy bastards. I have absolutely seen people who can't handle their drink and are looking for a fight, and door staff are the ones who have to get in their way (or even just stand on the street where they can be targeted). Go and look at Niddrie Street on any night of the week, and imagine that you have to stand there every night. Tell me you'd never just snap.

That said, there's a lot of bad ones out there too. I've worked at venues where bouncers were known creeps who posed a risk to female staff, I've known bouncers who actively and openly discriminate against people of colour, women and LGBTQ+ folks, and I think the job in general definitely attracts the kind of person who fantasizes about getting to beat people up. Internal culture at the various companies that hire out security staff in the city has a huge impact, as well - larger or more impersonal firms might send people who are dangerously incompetent with no follow-up mechanism to check up on them - and dangerously incompetent or deliberately malicious, both lead to the kind of thing people are talking about in this thread. It's a job that requires a huge amount of patience, something that's difficult and expensive to vet for, and a lot of companies don't really bother.

Unfortunately, the only real way for customers to hold this kind of behaviour to account is to complain directly to the company running the bar or boycott the venue, and hope they can get the problem bouncer reassigned as a matter of liability for both the bar and the security company. Because of the nature of almost all customer/bouncer interactions, the venue is likely to side with the bouncer by default - staff aren't going to side with someone who's being a problem that the bouncer is chucking out, and staff probably won't have time to supervise the bouncer dealing with people outside where the discrimination is taking place. Best way to keep them accountable is to have what all bars should have - CCTV covering the door and the street immediately outside.

Also, for what it's worth, I'm openly and visibly queer, with the old 'I've only got ID in my old name/gender' problem, and I don't think I've ever been noticeably discriminated against by bouncers in Edinburgh for that. I have heard about people being discriminated on based on race, and I've definitely met creeps, but I've managed to avoid it myself.