I haven’t been to a Regional here in the UK for a few weeks so I might be slightly out of date on this, but from what I’ve seen of the US Regionals it seems that i) there’s less CI; ii) there’s more AgInfusion, and iii) the Moons decks that are seeing play in the US are closer to HBFA than the tempo decks that are popular in the UK (i.e. Euro Moons).
Most Shaper decks have a bad matchup against Euro Moons, with the possible exception of the Adjusted Matrix kind. So if those aren’t there in the meta, Shaper is a better choice than it would be otherwise. They’ll also be better against more FA-heavy Moons decks, and should be fine against AgInfusion as it’s basically a fancy glacier deck, which you love to see across the table if you’re on the remote-lock Shaper plan.
The CI Shaper match-up is a weird one, but probably still CI favoured just from raw speed.
So basically if I’m going to a UK Regional I know I want to have an answer to CI and Euro Moons. Siphon Anarch would seem to be a better call there than Shaper, which is probably too slow for CI and can lose on tempo to Euro Moons. I imagine the AgInf matchup is pretty rough for Siphon Anarch, but I haven’t tried it. As there’s no Runner deck that beats everything at the moment, it seems fine to be favoured against CI and Moons, have game against SYNC, and give up the AgInf matchup.