I have had ladies who do this, mostly in group situations... One of the best things ever, when all is clean and consensual.
I'm lucky,it's damn awesome, my gal does not mind it and I got it everytime when she ask me to shoot it in her mouth, other than that it's always end in her ass. She's willing to do it because she knows that her ass was clean before performing any. Enema is recommended for this kind of situation and a willing partner.
My Ex and I loved doing ATM. It became our favorite thing along with me rimming her ass. We never had a messy experience and she never got sick after doing it countless times.
If it's kept clean there should not be a problem. The porn stars would not done it either if it's not clean enough for them.