If you have multiple accounts or your account has access to multiple tenants, you can only really work in one of them at a time. You have to log out and log in to each of them. Totally unusable.
Slack got this well from day zero. I can't understand how 6 years later MS Teams still doesn't support well this simple and common scenario.
At the very least, MS should let us run multiple copies of Teams, one for each tenant. The only way around it right now, is to have the desktop client open and then the web client for a different tenant.
I think philosophically the cause is that MS 365 and Azure AD are very much built on the idea of having isolated tenants with cross-tenant guests being local copies (the documentation for implementing Azure auth for apps explicitly recommends against mixing tenants) and tenant switching was clearly an afterthought. But there's really no reason not to allow having entirely separate copies of Teams for the different tenants just like you can have multiple copies of Edge signed into different accounts.
On the other I would be wholly unsurprised to learn that there are some obscure data sharing violations happening in the Slack client when you are connected to multiple Slack workspaces and Teams avoids them through rigorous isolation.