I work with Teams as an admin.

1. Teams is a resource hog.
2. Under the hood it appears to be a mess.
3. The user workflow aggressively tries to prevent users from multitasking, despite many jobs requiring this.
4. Product design is a shining example of the greater Microsoft philosophy of forcing customers to adapt their business to Microsoft’s technology instead of having technology flexible enough to adapt to a customer’s business needs.
5. From an admin standpoint it has numerous questionable design decisions surrounding UC. For example, you can register non-Teams SIP phones to it, but you can’t see the IP addresses of these registered devices(?!). In general, from a telephony standpoint, Teams is good at providing dial tone and some very basic telephony features, enough to make senior leadership think that it does everything, but if you need UC functionality for more than users who sit at a desk all day it falls apart quickly.
6. My conversations with representatives from the product team at Microsoft suggests they are out of touch with how businesses use telephones.

It’s way too slow for what it does. IRC clients from 20 years ago were able to switch between chats instantly. The user status (online, offline, etc.) is so inaccurate it might as well be random.