A more informative interview on https://theorbitalmechanics.com
This design allows them to feed 2nd and 3rd stage engines from 1st stage tank. After 1st separation, 3rd feeds from 2nd stage tank. All engines are a used, so reduced engine count on 1st stage plus more efficient as 1st stage is not lifting mass of 2nd/3rd engines.
Pressure fed, lower development cost, more reliable than turbo. Lower efficiency is counted by using all engines and them being vacuum versions.
Sea launch means no expensive launch facility with restricted flight paths. Operate off small ship, which can sail with wind so balloon is launching in still air.
Considerably lower launch forces on payload.
Not stated but the payload fairing can be light construction due to thin atmosphere.
The only area I wonder about is boil off of LOX and methane on the balloon flight.
Also working on balloon launched capsule for tourists. Issue I have with this is they are small not well funded company, need to pick one project and see it through to revenue earning stage. This is one area RL shine, laser focused on one thing.