Hsv 1 (oral herpes) exceedingly common, hsv 2 less so.
Hpv very common and many different strains, some which impart higher odds for cancer and some that don't, some that cause warts and some that don't.
All of these can be spread by asymptomatic carriers , i.e. they look normal but have asymptomatic viral shedding. Examining a partners genitals does not totally eradicate odds of tranmission.
Source i read claims most transmission is by asymptomatic carriers.
This trade/ hobby is not for anyone who wants zero risk, but then again such a person should probably just stick to masturbating as any of these viruses have a reasonable chance of being contracted from someone not involved with sex work.