The biggest thing is consent across all parties. You can absolutely cheat in poly. You can betray and be betrayed in Poly. And with the multiple relationships comes with having to be there for multiple partners at all times. So imagine how difficult a single relationship can be in terms of emotional and romantic commitment, it's multiplied by every partner.

But I wouldn't trade it for anything. Been married to my partner for 8.5 years. One GF for nearly 6 and another who was a long time friend turned GF for about 2 years now.

Having multiple romantic relationships at the same time sounds very stressful to me