If you're decent players and coordinate enough with your gang members, doing the HM is usually easy anyway depending on the amount/quality of the police on duty at the time. Cops being able to return unlike rebels sounds unfair, even I thought it was when I was a rebel however when you see it from both sides you understand that by the time the NLR is over for police, the trucks should've usually left the compound by then, at which time rebels can return anyway.
I could go into the fact that police in general have worse gear which is true, however rubber bullets, increased police force and the rise of AR squads going on patrol has definitely made bank jobs much harder than before and instead of blaming this rule of no returning for you losing the bank operation, think about how you could improve next time... better positions, tactics etc. I used to love shootouts at the HM Treasury as a rebel, even if it ended up in the police scrapping all of the trucks and my ass being taken to jail.