At the very least they would get access to your IP address (assuming you aren’t ok a VPN/proxy)
A public IP address is (by definition) public. If you’re behind CG-NAT you don’t get your own public IP and if you have a public IP but not a static one then restarting your router will change it. I don’t think there are many cases where an instance knowing your public IP is an issue. Lemmy instances hotlink media from other instances so many different instances get your IP just from browsing Lemmy.
My main concern would be instance longevity
This is a different conversation but if your account is meaningful then this should be a real concern. A month ago there were about 80 instances, now there are nearly 1000. How many of those will still exist in a year?