I've had to use Teams daily for the past two jobs I've had. Over-engineered is not what comes to mind when describing the app. It's not a complicated app, the UI isn't overwhelming, and if anything it seems like it lacks functionality.
Visual Studio - that's seems over-engineered (excellent app in many respects, but still...) The whole Lync thing felt over-engineered. MS has more than a few good examples of over-engineering, but Teams doesn't strike me as one of them.
I'm sure Teams has crashed on me at least once in using it for the past 3-4 years, but if so then it has been extremely rare - I wouldn't call it a crashy app. I've experienced performance issues with it, particularly when I've got my laptop hooked up to a larger monitor and I'm sharing my screen. There too though, I've learned to live with it and Teams works fine for me day to day.
However, I do see it as a lowest-common-denominator slack clone, and I agree with others here in that the overall Teams experience is lacking compared to slack.
It feels like MS is trying to do something different with it, but it hasn't worked. They keep pushing it so hard though that more and more people find themselves having to interact with it. Similarly, more and more shops pick it up because "nobody ever got fired for choosing Microsoft" and they're probably already neck-deep in Azure and O365 anyway.
Teams is not particularly buggy, it's not crash prone, it seems like it's missing some functionality, and other functionality seems like an afterthought (e.g., the wiki feature). It's just not a great app.