Under you legal GDPR/DPA18 rights you can request that all personal data on you is destroyed at any time. They have to comply with this.

It’s down to your own personal risk tolerance and how much you value the service vs what you are exchanging for it (money, data etc.)

Personally, if my only concern was data retention, I’d do one. I’ve read the privacy policies and the user agreements and they seem standard. If the terms change (and I’d be notified by law if they did) I can opt out and request my data is deleted.

No one is advertising to you based on your blood type (unless you’ve allowed them to.) Medical information is treated very very differently to say anonymised browsing data for Facebook ads. That won’t change until data protection regulation is changed.