At the end of the day, both large multispecialty groups and private practices will take advantage of you in that they will pay you probably less than what is "fair". However, the latter situation has the possibility of partnership, at which point you really can't be taken advantage of. In my experience, with most derm private practices it's pretty easy to tell if partnership is on the table . If they tell you yes and are specfic about the process, then it is. If they say no or maybe, then it's not. It's pretty simple. You will have the occasional situation where the time comes for partnership and they decide they don't like you enough to want to be partner. But I think a large percentage of the time it's just that you're not a good fit -- and not that they had a grand plan to screw you from the beginning (although that is sometimes the case).
I still think that the best option for most graduates is to just start a private practice on your own. At the beginning, it will be very hard and you will make a lot less. But after a couple of years of work you will be in a far better situation than if you had done anything else. It's true that there might be some highly saturated parts of the country where it is hard to set up, but those areas are rarer than you think. I think the biggest reason residents dismiss this option is that it is scary. There is no guaranteed salary and no one in residency teaches you how to really run a practice. These fears are compounded by the fact that graduating residents are in a lot of debt. However, these fears are short sighted. In most cases if you just take the plunge and start your own practice, in a few short years you will be much happier and better off than if you had done anything else.
Of course, the above only really applies to general derm. For subspecialties, it's an entirely different story.