When Blizzard makes an experimental card, they usually have multiple heroes changed. While it looks good for balance, because it is balancing multiple heroes in one patch, it has a lot of down sides to it as well.
One big downside to having many different balance changes in one patch is that the general indent that these changes left in the game is a little confusing. Let’s just say, for the sake of the argument, that Blizzard buffed Winston, and nerfed Brigitte in one patch. When people would play on the experimental patch, they would be confused as to whether or not dive is dominant because Winston is overpowered, or Brigitte is too underpowered, so that she is unable to keep dive in check.
Experimental cards are like science experiments, in that they each have a set amount of controls (unchanged heroes), as well as independent (Patch changes), and dependent variables (Game balance). In experiments, scientists generally have a rule that, at best, there should only be one independent variable. This is because, like in the aforementioned experimental card, having less independent variables means that you can have certainty that you will be able to tell that the changes that happened to the dependent variable are a direct result of the changes in the independent variable. Since these experimental cards are themselves experiments, Blizzard should operate with the same rules.
This is especially true, because if they took effort away from making many balance changes, and put all of their energy into making one, the balance changes that we could get could be much larger. Instead of just some simple number changes, we could instead expect to see, perhaps, some reworks to how characters function. Blizzard could figure out ways to fix some fundamental problems with certain heroes, like how Bastion is super good in the lower ranks, but really bad in the higher ranks.
I hope that this post comes across a developer, or two, because following what other people who do experiments do can really help improve the quality of the game, and make more fun for everyone.
TLDR: Experiments are preferred to only have one change of the environment that it is in, and Blizzard should prefer the same thing in their experiments.