I mained Mercy back then.
You’ll hear a lot of hyperbole about “Mercy hiding in a corner and waiting for her team to die” so she could get a mass rez, but in reality they almost never died like that.
In most games, your team didn’t die all at the same time in a neat little pile (requirements for a mass rez) unless they’ve been caught in a graviton or other ult… and that assumes that you didn’t get caught in the ult yourself.
They’d die at different times, and so spread out that usually Mercy would only be able to catch 2 (or more uncommonly, 3) in resurrect’s 15 meter range in any given usage of her ult.
Another reason why “mass rezzing” was an impractical thing to try for: it often ended up in your team dying again.
You know how the most disastrous use of Mercy’s rez currently is her drawing the attention (and gunfire) of enemies while she rezzes, dying almost immediately after casting it, closely followed by the very ally she died to rez?
Amplify that by 5, and you’ll have a realistic depiction of the dangers of using a mass rez.
Fun fact, flying in and shouting HEROES NEVER DIE at the top of your lungs draws attention… and it makes the enemy very interested in killing you ASAP. For most of multirez’s runtime, Mercy did not get any i-frames after using her ult. Making a “big rez” almost always meant sacrificing Mercy’s life to do so.
As for your freshly-resurrected team… the time it takes the rez animation to complete is enough time for the enemy to set up a retaliation ult without any fear of being shut down. Using a Dvabomb, riptire, high noon, or any other ult that takes some setup right after a Mercy makes a mass rez was pretty much free second teamwipe.
"Mass rez" is the overhyped support equivalent of Pharah’s rocket barrage: success with it was beyond rare, and it usually got the user killed.
Most Mercy players used “tempo rezzing” instead of mass rezzing.
That is, most Mercy players rezzed only 1-3 allies at a time, and only when they died despite Mercy’s best efforts to keep them alive. It was pretty much a jury-rigged burst heal.
There was a catch to using rez this way, though: you needed your ult to be available A LOT more often than the average Mercy. It meant you had to be popping off as near 24/7 as you could manage, in order to build ult as fast as possible - because Mercy’s ultimate was her only contingency plan against burst damage.
Sadly, this is not the way most remember Mercy’s rez ultimate.
In Feburary of 2017, the devs buffed rez to give Mercy i-frames after using her ultimate. This made it so Mercy could not be punished for getting “greedy” with her rezzes.
Some players realized this, and discovered that the SR system disproportionately rewarded “mass rezzes” - since most Mercy players preferred tempo rezzing, the average number of “players rezzed per rez ultimate” was around 2-3.
The SR system saw how rare it was to get a mass rez, and “thought” that getting a 5-man must mean that you’re OWL material.
There were players who abused this to get into ranks that they really shouldn’t. Performance based SR went all the way to T500 back then, and so there were players abusing the “mass rez exploit” to get as high as grandmasters with winrates as low as 30% - a perfect demonstration of how “hide n rez” was more more likely to throw the match than not… but with the massive SR gains it gave, some players did it anyways.
Instead of fixing the SR exploit or reverting the buff that made rez abusable in the first place, the devs decided to remove Mercy’s multirez entirely. I will forever be salty about that.