Tbh I think there are two types of dealbreakers and this is where the difficulty can lie.
The first are the things which would mean you wouldn’t date them in the first place and so it’s easy to enforce those because you don’t have the other parts of their personality to consider iyswim.
The second are dealbreakers which would or should make you want to end the relationship, but if they happen alongside other traits which you already know to be good then it’s not always that simple.
So e.g. for me I would never date a man who smoked, did drugs, had children or children they didn’t see, had a history of violence, was a bigot in any way, or had any kind of history of mh issues.
Within a relationship I would end it with a man who was violent, took up smoking or drugs, was controlling and showed poor financial skills/judgement.
But while I e.g. wouldn’t start to date a man who had any kind of mh history, it wouldn’t be as simple to end a relationship with someone who was diagnosed with any kind of mh during the course of the relationship as that relationship would already be established iyswim?
So on that basis, the dealbreakers you see going into a relationship may well change as that relationship develops hence why it can be so difficult to end things once they’re more established, whereas if you’d known from the outset you likely wouldn’t have gone there in the first place.