Generally, I feel there’s two things that determine whether something is ‘fun to play against’ or not.
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Effort/reward value
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Counterplay
This is why people generally don’t mind McCree, and have less problems with Genji or Tracer than with, say, Junkrat, Widowmaker and Brigitte.
Particularly Brigitte is a big offender on the first point. Stuns, disables, they’re fine in moderation. Mei’s stun lasts longer than Brigitte’s for example, but she’s lower on the frustration list. Much lower. Why? Well, Mei needs to track you, she’s much less durable than Brigitte, and she needs a second to set up the stun, rather than Brigitte’s instant stop.
You have some time to disengage from Mei once she latches onto you, and you have time to weaken her and even force her to retreat or freeze herself before she’s in range.
This applies less to Brigitte, who has an incredibly overweight shield to sit behind, and can dash to instant-stun you, with no aim required.
And then she can bop you for 155 without you getting a chance to play on.
Low effort, high reward is also seen as an issue with Symmetra (Although the lock-on beam’s limited range in conjunction with a no-mobility, low survivability character make her effort value hard to distinguish, making her more fair than she looks) and Junkrat.
Lack of counterplay is a big deal with Widowmaker and Hanzo, the latter now moreso than the former. A lot of characters cannot fight a Widowmaker, and will have no choice but to constantly cripple their own gameplay to avoid getting oneshot by her. Hanzo’s just a general mess at the moment.
There’s lots of characters that are okay to play against despite meeting some criteria for being ‘unfun’. Winston for example has a very, -very- low effort ratio when facing him, but, he also gives you plenty of time to counterplay due to low reward. Tracer has high reward, but also high effort. Hanzo -used to- have high effort (Outside of scattershot, which absolutely was horridly unfun) as a sniper and made for a much healthier character than Widowmaker. Zarya’s effort-reward ratio gets screwed when she’s high energy, but again, she offers counterplay in just not charging her.
Reworking a hero because of being ‘unfun to play against’ is a perfectly solid reason.