I have a potential partner who is part of an extensive poly network. Although we haven't gotten physical yet, we have already discussed where the boundaries and expectations are with regards to being safe when we do get physical. Those same boundaries and expectations are in place for everyone in that network (it should be noted that they are pretty much the normal boundaries one would expect safe sex practices). In addition, metamours will frequently be proactive about sharing their test results with each other. Basically, everyone in the network takes personal responsibility and common standard for practicing safe sex. This also means that if I were to pursue a sexual relationship with him, I would be agreeing to not only take active responsibility for my safe sex practices, but I would also be agreeing to only take on partners who do the same.
Since these are things I'd want to do anyway, it's not an issue for me. (I should also note that taking on such responsibility means that my partner should be completely comfortable discussing sexual history and personal sexual health and practices- if they aren't, that's a big red flag for me)
As far as how far it goes, even in a monogamous relationship you'll find that the web extends pretty far once you start including past partners and partners of past partners. In a poly network, it's just that some of those partners are current rather than past. In some ways it can be safer because it's a lot easier to track down and communicate with a current partner about something that may come up than it is to talk with past partners who are long gone.