There were some problems with the early Jabiru engines, but it seems like most of that has been worked out for several years now. However, because the engine is six cylinder carbureted, there can be issues getting temps right. Seems like most builders have to work on that in the beginning, making adjustments to intake and cooling airflows to get it dialed in. My general advice on engines is to not buy too early. It is very easy to think that you are going to actually need the engine a lot sooner than reality. In other words, try to avoid having a new engine just sitting in the shop for several years.

Honestly I plan to go with the Viking 195T for my 650

The Gen 4 Jabiru is a solid choice.  Engine installation is simplified because you do not have fuel return, header tanks, electric dependent engine controls, or a radiator.  It is a simple old school horizontally opposed six-cylinder engine.  Smooth running and no cooling problems with the Gen 4.  Also, the firewall forward package from Arion Aircraft includes everything you need except the master solenoid and the battery.  Worth comparing to other installations that need more items that are not included or are in a separate package.  If you need install pictures for the Jabiru or have trouble finding the install manual feel free to reach out to me when you get to the engine install. Good luck with the build.