I can only speak to my own experience with my sufferer. He has cPTSD due to neglect and abuse throughout his childhood, and a second diagnosis of PTSD from a work-related injury. I haven't been too active on here of late, but my story is on this site, if you're curious for more of the background.
My short answer? Yes. I think it's very difficult for someone who has cPTSD from a neglectful and abusive childhood to form lasting, healthy, intimate relationships, especially without treatment, counseling, and possibly counseling together. Romantic and intimate relationships (I'm including close friendships here as well) are difficult enough as it is, without mental illness in general, and without a mental illness that hijacks the emotional and instinctive brain centers. We haven't yet mastered being able to read minds, and communication of any variety is imperfect and sometimes difficult.
We learn how to interact with other human beings as children, and especially learn about intimate, close, and romantic relationships from our parents (or other caretakers - the people who are "supposed" to be in charge and taking care of us) - by watching them interact with each other, other people, subsequent spouses, and by how they treat us. Throw in a narcissistic, neglectful, or abusive caretaker, and your first examples of human interaction are, well, painful garbage.
My sufferer was abused, mentally and physically, by his parents, his subsequent step parents and families (from both sides), and by the people outside his family who should have been looking out for the kids entrusted to their care (including child-focused counselors and a psychiatrist). He watched his parents and step parents abuse each other as well. He had no examples of what love or a healthy relationship looks like, and only had examples of what it most definitely does not look like (pain, neglect, and abuse).
So, during the time when he was supposed to learn how to interact with other human beings, he learned other people can not be trusted. He learned shame. He learned that "love" (ie, what the people in his life claimed was love) hurts. He learned that even the people who are supposed to help you when your home life goes to hell, can't be trusted either. And, in reality, he didn't even learn how to HAVE emotions in a healthy way. If he had emotions, he was ridiculed and/or beaten.
Adulthood hasn't necessarily brought relief - he still refuses treatment, and it wasn't until the last 9 years (as long as we've been together) that he realizes that maybe there is hope. He "ended" our relationship (his third marriage) a year and a half ago, but is starting to realize that love exists, and doesn't have to hurt. But he also realizes that he has to learn how to have healthy emotions, and how to compromise with another human being who doesn't actually mean him harm, and how to not only let someone in, but to give enough to the other person so that he's not just taking. He has to learn the tools to deal with the brain that was mis-wired in its development by abuse and neglect, so he can start to function normally.
I think it will be a struggle for him for the rest of his life. And, he has to be willing to do the work to get there, and that work will be hard. It will require outside intervention (from a trauma therapist). And, he has to realize that only he can do that work, even when he has a partner willing to walk with him, and even if he can find a good therapist. Healing is possible, but it's not easy, and it takes a willingness to even admit healing is needed.
Human interaction is hard for anyone. It's especially hard when interaction requires bridging a gulf of fear and trauma. Healing to the point of being able to be a partner and not just a presence takes courage and tenacity.