From a practical point of view, I don’t really see any downsides.
Studies have shown that you see worse online prices than I do, because they know what you are going to buy next and they have a good guess the maximum amount you are willing to pay.
VPNs aren't enough to solve this, but you may be unaffected if you have other strong anti-fingerprinting habits, including routinely deleting all cookies and disabling JavaScript.
A terrible The worst thing they could be doing is providing an annual report to potential employers on how much money you make, helping your employers minimize how much they pay you. Of course, Visa could also be doing this.
The other thing they could be doing is sharing your health history, ethnicity, and any religious or political leanings with potential employers, health insurers, or governments.
In certain cases (cough the USA) we know the government has a legal right to a wire tap, if any of the servers are inside the USA.
How much any of that could affect you long term depends on who has power where, of course.
Sorry to drop all this for you. It got to where I didn't feel the risk was worth it for free email, for me. I wish I had jumped to a privacy respecting solution sooner, since I don't know how long my data will take to age out of their systems.
Edit: Sorry...I realized I used the word "worst" pretty lightly here. Worst is more like...https://en.wikipedia.org/wiki/IBM_and_the_Holocaust