GoOpti is well regarded here and on the Slovenia forums I frequent. I would use them in a heartbeat if need be, but I've not yet have the opportunity.
They aren't really like Uber or Lyft. You have to schedule them in advance, and the price probably goes up the longer you wait. But you can't just open your phone and request a GoOpti at any given moment and expect them to show up. They own their vehicles (it's not a "ride-sharing service") and schedule pick-ups based on prior demand. They do have shared transfers as well as private transfers (what you seem to be considering) - the shared transfers cost a whole lot less than $100 per person.
With frequent bus service now between Venice and Ljubljana (e.g. FlixBus), I don't see any need for a private GoOpti transfer unless you really don't want to deal with a bus and want something private and prompt. The train now runs between Venice and Ljubljana with a connection in Trieste; that's at least an extra hour longer than the bus and requires the connection in Trieste, but it is worth considering. I don't care for long bus rides, but a direct bus would be easier and faster than the train.