It takes away from everything the product is trying to do by focusing so intently on misguided flaws of the competitor product. It's like those Galaxy vs iPhone ads.
Make the product better.
This article read like an advertisement. There's nothing wrong with advertising, but this isn't even a particularly good ad.
I'd be more likely to try whatever the heck this service is called if they talked about what they do, and focused less on calling out Slack.
The problem with ads that shoot down your competitors is that your readers are busy thinking up counterpoints when they should be learning your points.
The first car with seat-belts had to explain in its advertising just how existing cars are dangerous, before it could explain how seatbelts fix that problem. People would start off thinking that existing cars are just fine; the ad needed first-and-foremost to make them believe that they weren’t; to foment a state of irritation or even disgust at “the way things are.”
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Not to say it’s always a real problem, necessarily. You can convince people they have a problem whether or not they would have agreed they have a problem if they were just provided with some additional straight facts.
See: the invention of deodorant. First take something society doesn’t actually care about, even if it’s pointed out—the fact that you smell sour after a hard day’s labor—and make it into an etiquette faux pas for people to be able to notice that about you in the public sphere. Then you can sell the solution.
Admittedly, even this manipulative version of the technique can still have unintentional positive knock-on effects: public transit probably never would have been bothered with as a concept without you and the sardines around you on the train being deodorized first.
As for the rest of your post, you're right, though you can introduce a problem without devoting hundreds of repetitive words to bashing a competitor. Talk about how hard it is to use chat in remote teams, or talk about any of the other problems with chat. That will at least get people to agree.
In this ad, greater than half the content was devoted to bashing Slack. At best, that's unprofessional. At worst, it's a very poor strategy.
If I were Slack, I'd keep an eye on whatever the hell this company is called and build the feature of it catches on.