I hate it when developers make what they say are side games in a series essential to canon, especially when they don't tell you they're doing it or when they go into the side game not planning to make it canon and then decide it's canon during or after development.

Or I should say, I hate it when developers make the next main series game assuming you've played the essential "side" game and leave out or half-ass their catch-up for people who haven't played it. I call it "Chain of Memories syndrome" after when Kingdom Hearts made Chain of Memories essential to fully understanding KH2.

Having played Portable Ops (fine), Peace Walker (fine), and Acid 1 and 2 (FUCK YEAH): 5 was still mostly nonsense.

All you get from PW/PO (since PW largely felt like a redo of PO and PW was a prototype for the format of TPP):

1. This random lesbian was in love with The Boss but ended up fucking Huey and is Otacon's mom. And somehow that is the least problematic portion of her arc.
2. Paz is basically the jailbait version of EVA in that she is a spy who betrays you but also everyone wants to fuck her but her true love is, and always will be, Big Boss. And don't ask how old she is because nobody wants to know that.
3. Chico is your loveable sidekick that you never really gave a shit about but he is like 12
4. Big Boss has either stopped or started using Big Boss as a title for the umpteenth time because we can't play as someone not called "Snake" and has something that may or may not be Outer Heaven built on an oil rig.
5. Don't think too hard about why Skullface is basically just Hot Coldman but more of an edgelord.

A friend summed it up perfectly: Ground Zeroes works a LOT better if you pretend that Peace Walker didn't exist and this is just "the adventures of Naked Snake". Similar to how we never really know what Solid and Otacon did as an NGO before they became international terrorists in MGS2.