Interesting question. I can't believe that I can't think of any other threads that address this question. Even though Mono and I have talked about it. It is likely not evident in other threads what the answer is to you question, but if you are interested... you could do a search in the tags for "mono poly" "monopoly" or the like... there is a lot of info... although not specific to the question. There is little on the threads gone before on either mono or poly being better over the other... just so you know.
Benefits to mono.... you get to be the one and only person of someone who is not going to wander, have eyes for another, seek out more sex or love from another... everything is about you in every aspect of their emotionally connected life. At least hypothetically
I believe that some people can actually pull that off, or are actually built this way. Its like a dream to me as a person who thrives on a lot of attention and a focus for her love in return. Others find it tiresome and a weight upon their shoulders when someone is that focused on them. It is certainly a lot easier and one can concentrate on something other than their relationship life... at least in terms of my life anyway.
Mono is an easier ride for many as it is handed to us... the rules are set there for us to follow. We can rely on them and fall back on them... that feeling can be a relief sometimes, yet it can also mean a lack of adventure and become stale in a relationship.
Benefits to poly..... you can spread your love around or experience your partners love being spread around. That can manifest in some fun sexual experiences to some amazingly close friendships and community building with people who are like minded and just as willing to love you as your partner. It can be an adventure that pushes most people to the edge of their comfort, only to find that they are capable of huge amounts of acceptance of others and self love.
There is abundance of loving feelings when poly is done in such as a way to expand on that feeling... sometimes it isn't or someone within your realm is not capable of accomplishing that. When it's going well, it is an incredibly world expanding, holistic feeling. When it isn't going well, it feels incredibly vulnerable and over extended.