We strongly recommend having a dedicated vent/overflow connection for each bag in the system, especially when bags are to be hidden in storage lockers as many installs will be. That said, if you fully understand the reasons for that dedicated vent line (pressure relief and air purging), ultimately it’s up to you to weigh the pros and cons and come to a decision that works best for you.
One of the big advantages of a fully automated system though is that it allows you to fill while driving the boat across the lake, and that makes it incredibly easy to start filling the system, and leave it running for long enough that the bags are full and the pressure in the bag gets to a point that is dangerous.
If you know for sure that you will be able to monitor the bags during filling 100% of the time and you would rather not install the thru-hull fittings for venting the bags then an unvented system can be used. But we’ve had numerous customers that have installed systems using reversible pumps (that generate higher pressure than aerator pumps) without vent lines that have done significant damage to the trim panels in the locker and fiberglass/gelcoat of the deck of the boat. We understand the reluctance to drill additional holes in the boat, but our belief is that there is a right way to do everything, and the correct way to install an automated ballast system is with a dedicated vent connection for each bag.