A full Geo team is very dominant in the early game because it's one of the first teams you're likely to encounter that requires a specific counter. Once you have that counter, getting through them is completely trivial. It's key to do well in early arena to develop a solid, long-term counter and a Geo team of your own.
The counter should be something like Vader or Traya that will last you ages and shouldn't be something you need to beat other teams at your level -- and the Geo team will contribute to you getting into very solid guilds early on because Wat Tambor is such a valuable character now, both as a rare GL component and as a solid piece of many late-game teams.
And Geos are a key component of one of the most dominant fleets in the game. You really can't go wrong with them.