I would guess that poly relationships where one partner prefers monogamy, but is "settling" for polyamory to be with a particular person, are not likely to work well (and vice versa).

I've seen several relationships like this (both the "poly agreeing to be mono" and "mono agreeing to accept poly" variants) and you're right, that does tend to create more or less tension in the relationship. In some cases the partners do manage to adjust and everyone has a rather happy relationship, in other cases it's just a question of time when this difference will make the relationship fall apart.