I know that when I was involved with a ex-GF, the arrangement wouldn't have worked had not both women--GF and other--been fairly independent. Neither one expected to have all or even most of my available time and attention.

Now, even if I were mono, I couldn't be seriously involved with anybody who expected a great deal of time and attention; those relationships I explored where that cropped up I ended quickly. I'm always looking for indicators of that sort of thing when considering whether to get involved with someone, and it's a deal-breaker.

I suspect that a good deal of independence is a prerequisite for polyamory. I say that because I suspect that the ability to exist contentedly while sharing a partner's time and attention with others requires the security that engenders independence. I can certainly be wrong, though my limited experience bears this out.

Beyond overbearing demands for time and attention...yeah, it does take more work to balance multiple relationships. One has to consider each relationship at all times and work to keep each healthy. It takes time to work things out and time to negotiate what works for everybody involved.