On Holm, let's be clear: she is EASILY the most technical and accomplished striker in the division. In that respect she is a completely new type of challenge for Rousey.
As Jack Slack argued, the key to beating Ronda has to be neutralising her strengths - which are too lethal to overcome directly. Thus, you have to deny her the clinch. Circle out. Use lateral movement. Slack argued that Rousey tends to get the clinch by coming in swinging and tempting her opponent to bite down on her mouthpiece and throw back. The problem is (i) Ronda hits hard - ask Davis and (ii) she'll just grab the clinch anyway and throw/trip. From there the fight is generally over instantly, unless you're Miesha Tate and can survive for a while.
Her opponent needs to be able to resist the temptation to throw down and instead have the discipline to try and circle out/use lateral movement when Rousey comes in looking to set up the clinch. IN THEORY Holm may be ideal for achieving this: boxers have to know when to stick-and-move rather than just brawl, it's integral to the whole sport, and also who has better lateral movement than a boxer?
IF - and it's a giant IF - Holm can deny Rousey the clinch then she has a chance to win the fight. Her striking is superior (though not as powerful as Ronda's, I don't think) and way more varied. She has hard body kicks and good combinations. Jack Slack suggested kicking Rousey in the knee, Jon Jones style, as nobody ever catches those and they slow opponents down. We don't know much about Ronda's Plan B or even her cardio. We don't know how she'll react to frustration. We don't know if she'll be able to adapt or just keep trying Plan A over and over.
So, IN THEORY, Holm probably has more tools to test Ronda than other Rousey opponents.
In practice, however, I'd expect Ronda to win within two minutes, submission or KO. Her Plan A is just too dangerous and she implements it magnificently. In addition she has the striking power to knock people silly, even though it isn't technically perfect.
