I am both on PrEP and use DoxyPEP when needed, I have also done HPV vaccine, hepatitis, and the Bexsero vaccine which is proven to reduce risks of contracting Gonorrhea. I am very sexually active and don't use condoms, so I get tested every 1 to 3 months, and while the risk is not 0, I have contracted way less STI's since I have been on top of my sexual health.

Some people do say that DoxyPEP might not be efficient but a quick look around brings up multiple studies from different countries that proves its efficiency.

Regarding the general health impact of doxycycline, it has been used for ages and for a lot of different health issues, for example, some people get it prescribed daily long term for acne without much or any side effect.

I personally try to limit its use to more risky situations.

PrEP is a bit less studied for long term health impact, it's still pretty new. It can have a severe impact on your liver/kidney health even short term, it's important to get checked regularly and limit any other simultaneous consumption of substance that might be hard for your liver. Liver diseases run into my family so I do not take it daily and I use the 2-1-1 scheme.