The biggest issue: you cannot not give them your personal data that they then loose.
Let me contribute with an anecdote from yesterday (slightly off-topic but I promise to get around to it at the end). So just yesterday I needed to create a Microsoft account to try out Teams which is supposedly free. (I have avoided it so far, but my GF has been asked to use it for an interview and we wanted to do a tech test run before). Of course, the UI on the website assumes (!) that you already have a Microsoft account. It will let you create a Teams account that will fail the login if you do not have a Microsoft account and then sends you around in a Byzantine loop without telling you: Look you need a Microsoft account to use Teams. It looks to me as it just creates a shallow alias or something without root reference. This is dark patterns all over the place.
Anyway, a bit more on topic, I am course using my spam email for this account, but then they ask for my phone number. This is really an issue, because except if I get a burner phone, my personal data is linked with an account of a company I do not trust. After witnessing then how bad teams is almost 1.5 years after everyone is working remotely, (wow their web client does not allow you to share webcam and a window/screen at the same time, while their native client makes it super hard to share content while still seeing the people who you present to), I realised
1. How privileged I am not having to use Microsoft products (need to remember to charge extra, whenever asks me do a job that involves Microsoft products)
2. How anti-competitive Microsoft still is (you cannot login to Teams, MS web auth, in Chromium incognito mode, and it needs a ton of cookie domains whitelisted, even then it does not work)
3. How (and this is not Microsoft specific) difficult it is to not hand over personal data to companies that provide a utility-like service that they pretend is free (so everybody can pretend they are inclusive when they use these services)
4. An then literally a day later it turns out I am not paranoid not trusting Microsoft (and I guess other companies, big or small) with my data, because they are going to loose it sooner or later.
Edit: I just logged back into this MS account. They dont even use the phone number as "2FA". They only send you a text when you register, not for subsequent logins. It looks to me as they just collect it to make sure they really have some personal data to loose..