There are countless way I would consider the game broken. Many of them too small to remember or notice except in specific situations, which is part of the problem. Because Blizzard considers itself very clever to implement many, many little tweaks and subrules, which change straightforward expectations and are basically impossible to account for. For example, it seems a timer of a checkpoint can speed up considerably, which can appear as an “instant” to a player being a couple of meters away while all its teammates gets killed by a bastion ultra. There is no chance to get back more quickly than an “instant”, as one might suppose, and yet apparently this is already to directly “switch” to a defeat/end, although the other team wasn’t even particularly dangerous (nor the timer advanced) and it seemed they could still be pushed back.
Then there are also many moments when a tank gets basically “one-shotted” by apparently normal standard attack without it being really apparent why. Aside from that I doubt the correctness of this possibility, which does happen a lot sometimes (with normal care and use of defense when one has time to react). Especially Moira I would consider a “culprit” of mysterious “instant-kills”, which seem hard to reproduce and apparently calculated by such things as closeness and whether one is already hurt (speeding up her attack, rather than leaving it at normal strength).
This concerns random little tweaks and sub-rules which quite randomly complicate the game, and which seem to be deemed overly clever by the developers themselves although they are of doubtworthy value for game quality (except for those posers who blindly support any “toughtness”, even the most arbitrary ones).
The other part concerns certain exploits of characters. In certain character combinations one character is at a strong disadvantage to the other, making it pretty pointless to even try (one probably will because one has to, but it’s no fun at all and leads to a high degree to automatic death waiting sequences). For example a fast, hard to hit character like Tracer can be impossible to catch for a Phara (both fairly close together), and yet Phara has just as little health, technically even less because no healing. The potential of exploiting certain characters is tendentially higher than for others, with little one can do about it (except to die and hope for another character to become free). This overspecialization is another element where the developers consider themselves very clever, but which can actually skew the balance rather than provide it. Another example which I’ve found has become rampant recently is a bouncing Lucio, waiting at some edge (I neither know how he can “bounce” so much or how he can disappear so utterly as he does sometimes, but be that as it may) and pushing anyone coming near into the abyss. This might be fine “sometimes”, but this is another instance of an exploit. In some maps one is actually at a risk to be driven near a edge, and one is basically powerless if one someone uses such an exploit.
I think the game is unbalanced by many different, even tiny things, so tiny and unexpected I bet nobody really is conscious of all them, and things Blizzard seems to consider particularly clever. Let me explain that by “unbalanced” I basically mean “not worth playing”. You might just reject my complaints, by deeming yourself clever after all (and anyone else by default on the internet), but if the experience is not worth it, regardless if you think yourself clever, it is kind of broken. And some things (like odd timer stuff or intransparent insta-kills) are of doubtworthy consistency with expected rules.
This is just a futile attempt of a topic too large for a single person or a single moment, and without actually having access to some of the technical data of it.