Depends on your internal world.
I find the phrase Unicorn Hunting an interesting one. It seems to be associated with a slough of ideals and expectations, and often ends up creating scenarios where people are way outside of their emotional depth. I think that for many, it's a natural start to the ideal of opening up a relationship, and that it evolves over time. I had been poly in lots of different ways before I became curious about sharing a girlfriend with my husband, and had insanely good luck in finding meaningful relationships with 'unicorns' pretty much right off the bat - I'm guessing this isn't the norm... don't know what made our situation different. I see the women that engaged in those relationships less as some version of a mythical beast, and more as kinky women who wanted to share friendship and sexual intimacy with two other people at the same time.
Our problems reared their head like beasts straight out of the depths of hell when we tried to make it into something that it wasn't. Trying to figure out how to move us all in together in a poly-fi-triad where everyone loved each other the same? That was the pinnacle of naivety and stupidity in our love triangle experience, especially when we tried it with a third that had little in the way of relationship experience and communication skills, and we were all in a haze of NRE and maximum-spaz-out mode. Ridiculous. Catastrophic on all accounts. It almost ended our marriage, and sure as shit fucked us all up. Royally.
The most successful triad relationships we've had were ones there ideals and expectations were checked at the door. We have an awesome time when everyone stays present. When things move slowly, cautiously and respectfully of all involved. When everyone is damn clear about what they have to offer, and what their limitations are. Our girlfriend now is having fun, experimenting sexually and enjoying the intimacy, friendship and excitement of dating a couple, but she's made it damn clear that while we're sure to stay friends, that if the right guy comes along she is out. We have dyads. We respect and communicate with each other about concerns that come up as they come up. We are friends as well as lovers, and check in with each other about our days and happenings in our lives. We do NOT talk about the future of our relationship as though it's a give in, delude ourselves into think that we're all going to feel the same way about each other, or view any of the parties involved as less than a whole person who has individual rights, needs and desires worthy of attention and respect.
So is it a bad idea in its essence? I'd say that it's no different than the likelihood of any complex relationship thriving. Depends on your mentality. Depends on your ability to communicate, stay present, stay realistic, stay grounded, stay centred. Depends on your ability to check your ideals, expectations and fantasies, and really see things as they are, including your own limitations as a partner within relationship. Depends on your ability to pace, move slowly, and not get ahead of yourself. Depends on a lot of things.
Don't let other people pre-judge your potential reality based on negative experiences in their sphere. There is no surer way to get away from your own hearts, your own truths, and find the path, and the poly/openness that works for you. You speak of your ex-unicorn's friendship and respect, so it sounds to me like y'all did something right. Over the past three years I have had really great, and really shitty experiences with a third. I have been a third to a gay/bi male couple, and will be friends with the surviving partner (the other died in an accident) until the day I die. I think that there is real value in triad relationships, and that it is isn't an inherently flawed relationship structure if everyone is conscious and conscientious. People want to experience them, and 'unicorns' want to experience them - people get hurt in their context, but people get hurt in the context of ALL relationships, not just triads. For us, it also led us to open up our marriage to the idea of dating individually, which is something that we hadn't initially thought about. This is your journey. Live it with integrity, and don't let other people define it for you, or stereotype you.