Being careful and being successful are two different things, especially given how hard it is to do an exhaustive search over patents to ensure that no, no claim in any patent filing touches your code, especially given that there are regimes where patent filings for software get pretty much a rubber stamp and are based around "first to file" even if there's prior art.
I don't think they'll be successful. The Alliance for Open Media was careful to avoid potential patent problems during AV1 development. So, unless AOMedia seriously failed in that effort, AV1 will be alright.