Sombra was always an interesting hero in Overwatch but at the very beginning, she was quite weak. After a short time, she received a nice buff and that forced players to pick this hero more often. Now it is a common thing to be EMPed or hacked. So I think that you need a decent guide that will tell you how to counter such an annoying hero as Sombra.
But first, you need to understand the basic things about the Sombra and that knowledge will give you all you need to claim an advantage when you are playing against this hacker.
The most common thing – as most of others flankers she will always appear behind your team, and, most likely, she will set a translocator to the nearby med-kit. The best you can do in this situation is to always be careful and be ready to peel her off from supports. You will not kill her, but she will be scared and will use teleport. If she will not be able to kill a target, then Sombra will be a useless anchor for her team. You will not spend a lot of time to scare her from her ambush, but she will be forced to wait for the translocator cooldown, so you will buy for your team window that is 15-20 second wide. The faster you will scare her from the point, the better, after all, she will be too scared to even attack from the distance and that means that her ultimate (the goddamn EMP) will not be charged at all.
But be careful, especially if you are playing as a hero who cannot do much when he hacked, like Lucio, Tracer, D.Va, Zenyatta, or Doomfist. So in this case, you need to avoid her at all costs or interrupt the hacking process if you have the necessary tools. Interrupting is hard, especially considering the extremely fast cast time, and only a few heroes have crowd control abilities that could be used to disable Sombra. That is what makes Brigitte and Moira one of the best heroes to avoid Sombra – first one could beat her without any help (and interrupt hack with the shield bash), the other one will always have Fade ability to run away from any threat. Always keep your mind clear and keep her location on your mind. You need to be fast to react to her actions, that is why if you are playing on a Defending point map, you need to stay a slightly afar from the rest, to not be affected by the EMP. Like if you are playing as a Zenyatta, you should keep yourself hidden, and then, when Sombra will use EMP you need to counter it with the Transcendence.
Tracking Ultimates is the matter that is worthy of a separate article, but here I will highlight it only slightly. Most of the professional players from https://expertboosting.com are perfectly predicting the ultimate charges, that is why they are so perfect in this game. If you will be able to track her EMP charge you will be always ready to counter-react for any possible outcome. Keep in mind that EMP charging speed is insanely quick and you need to expect the worst from any EMP usage. Like combo EMP + Tactical Visor will guarantee that everyone who was hacked will be most certainly dead. But do not panic, most of the players of low divisions are just scare of EMP too much, they even forget that you could still attack while you are hacked. But if the whole team tries to focus the Soldier 76 or McCree, then you could minimize the possible damage.
Playing as Sombra you need to correctly evaluate every hero in the team and use your Hack and EMP on the most high-value enemies, only such an approach will lead you to the victory screen. Sombra does not like pressure at all and will escape with the first sign of a threat, that is why without the proper coordination with the rest of the team she will not be able to be as impactful as she may be. If you have a plan, then Sombra would be extremely strong, but make sure that your team knows your plan as well.