I gave two stars because this is a hostel, so leeway needs to be given. I wasn't aware that Selina is a big corporate hostel company (there was a culture to the experience that was odd, the welcome meeting included self-proclaimed traveling "witches" and energy healers giving unwanted advice...no joke...but it's a hostel so I get it'll draw a certain crowd).
The first night a guest in my room was screaming in their sleep, for 7 hours - I never experienced anything like that, almost traumatic. At 4am, I couldn't take it any more, told the night supervisor but they were not quick to remedy the situation. They eventually put me in a random room with unmade beds to sleep for two hours; I just fell on a blank mattress in hopes of sleeping one hour. They told me to come back in the morning (which it was already) to get help for switching rooms; when I went back, they didn't have any rooms available. Dizzy from no sleep, I was losing it, and this was no condition to be in when traveling. No way was I going to go two nights with zero sleep listening to a person scream like they're dying for 7 hours. After a lot of conversation with the lobby desk people, they eventually moved the screamer (probably because someone else also complained). But then there were multiple room assignment mix ups so random people would come into my room upset they didn't have a bed (so imagine being sleep deprived on day two and then having strangers come in and out questioning you about what bed is yours etc; Selina was overbooking or something, it was chaos). Then, the room key cards stopped working so we had to constantly go down to the lobby to get them fixed.
There was more, but I don't want to write a Selina horror novel. Instead of fixing multiple issues, there was an underlying attitude of "we can't do much, unless you want to upgrade to a nicer room."
Probably the most stressful, unrelaxing stay. It clouded my perception of Mexico City until I moved out of there. Mexico City is wonderful, I wouldn't let this place affect my experience again.