A lot of street homeless people have serious problems, mental health, addiction, just left prison and straight onto the streets, etc. There's also, now, a lot of people who have no status, no papers, no connections, too.
That said, they are, fundamentally, just human beings, and there is good and bad in every group. They're generally not evildoers, but sometimes, addiction and stigma can make people behave badly. Quick fact: most UK natives who end up homeless were once children 'looked after' by the State. There's no safety net for these kids, and it's easy for them to fall into drug addiction and homelessness. Most of the serious drug addicts (class A injector types) are groomed into becoming addicts, just like you groom children into abuse. Supplying drugs to addicts is a very lucrative business, and just one proper drug addict earns a dealer around 40K a year. Five £20 bags of brown a day, seven days a week, 52 weeks a year... do the math, if you don't believe me.
Once people are on the gear and homeless it's bloody hard for them to exit the lifestyle. Nobody gives a stuff about them. You don't see the scared and traumatised kids in them you just see the anger, aggression, desperation. If they can take advantage of you, they might, but they don't all behave like that, and the ones that do are doing it as a survival mechanism rather than because they're evil. A lot of people develop this 'us and them' type mentality, and you yourself seem to be demonstrating that, too. You don't really want to work with 'them', but you're willing to exploit them and their circumstances to get where you need to be just like the majority of other 'professionals' they meet on a daily basis, and just like the dealers who sell them their gear. Sad as it is, exploiting them for your own gain won't make you stand out. You'll just be another faceless 'staff' member. They will care about you as little as you care about them.
The trick is, though, not you seeing the humanity in them, but getting them to see the humanity in you. From a personal safety perspective, seeing you as a human being could one day save your bacon.
Human decency, and decent boundaries. That's what you need. Not phoney caring when you don't, but a basic interest in other people is useful. If you can't bring yourself to treat them the same way you'd treat anyone else, if they get a sense that you're judging and looking down your nose at them, you will be slotted into the 'them' category. If they appreciate that you're honest and not too soft, then you'll be fine. If you take an interest in them, as human beings, learn people's names, treat them like actual people, then... you'll likely be okay. You might surprise yourself and find that yes, it can be a bit rough, but actually, these are people managing to survive in circumstances that'd crush most people and given half the chance, most of them are okay.
You will probably encounter the occasional cunt. I would be lying if I said otherwise. There are cunts who will bully, harass, intimidate and try to violate your boundaries. But that's not a feature of homelessness, or addiction. That's because they're cunts. They treat their fellow service users the same. That's when your professional protocols are most useful. People can, and do, get banned from services, denied access, etc. Know the rules, and don't be afraid to state them as facts. Everyone plays by the same rules. There are no exceptions.
The basic stuff applies: don't share your innermost secrets with anybody, don't lend money to anybody, don't tell people where you live, don't make promises you can't keep, and don't pretend to care if you don't. Don't wave your expensive jewellery in people's faces, secure your possessions, observe the organisations protocols regarding lone working and security. And... in turn, don't be a cunt yourself. You're not the police, you're not a counsellor, you're not there to 'fix' them with your genius. You're just there for your own reasons. The same as they are.
I don't think working with people who are homeless is necessarily any more dangerous than walking through the town centre at 2am after the clubs kick out, myself. If you would find that arduous, then perhaps find an 'easier' placement.
Have worked in homelessness services... and have also been homeless myself.