Just for 2021 and onwards a new view on this topic:
While Twitch and Youtube (and Steam Broadcast) still focus on massive amount of viewers (obviously), latency still takes a hit with that services. Apparently there are ultra-low-latency modes in Youtube and Twitch (look up this link) and they are "fine" but it's still meant for streaming for a big crowd, so it's still not the best way do play Jackbox Games.
Discord supports screen sharing in private calls and in voice chats on servers. These are still limited to few people (compared to Youtube and Twitch) but enough for any Jackbox Games round. As they are so relatively limited in terms of viewer count, it allows the stream to have way less latency. This is probably the easiest and best way for anyone to play Jackbox Games over the internet.
In contrast to Steam Broadcast, the 2019 released service Steam Remote Play streams not only the screen from the host to the viewers but also button inputs from the viewers to the host, as it is designed for any local coop game to work over the internet. The input transfer is not important for Jackbox directly as the interaction from the viewers works over jackbox.tv anyway, this fact does show that this service is completely focused on ultra-low latency screen sharing because that's what's needed for playing local coop games online, even more than for Jackbox Games. This puts Steam Remote Play maybe on an even better spot than Discord in this field. Nowadays it can even be shared through only a single link, meaning not even a Steam installation (and afaik not even a Steam account) is needed at the viewers/clients end.
Google Hangout apparently also has very low latency but I haven't tried that yet.