The problem is that there are no real disadvantages that come with her flaws. Bad family? Well, Ian and Lena are here to create a happy family with her! Self-esteem? She can act around as she likes when she is with Ian/Lena, they accept her. Doesn't know Ian? So what? As of now, there is no disappointment, no criticsm on Hollys part. She goes along with whatever Ian does.
Holly doesn't complain, doesn't argue, doesn't break up, doesn't have plans and mind on her own, doesn't contradict her partner, doesn't set conditions. And despite all, the relationship goes smoothly, has no downsides. Ian only breaks up if he is the one having too much issues with other women.
The other women have ego. Treat them like shit and they react accordingly. Ian can even mean well and they can still feel insulted, for better or worse.
Of course, Holly is the ideal woman with porn mindset: She makes no demands and is always sexual available.