As far as I know a 6BT-5.9 is a Cummings engine. Cummings makes good engines. That engine is a light to medium duty engine. Meaning it can't be run at rated hp for more than 2-6 hours in a 12 hour period.
Not to split hairs here; close, but not exactly so. The Cummins (no "G" in Cummins) 6BT is made in a range of configurations, among which you'll find continuous, heavy duty, government service, medium continuous, intermittent, high output ratings.
The 180HP 6BT in my boat is rated at medium continuous. That rating limits full power to 6 hours out of every 12 hours of operation where the reduced power is at least 200 rpm below the rated full power rpm, and intended for use accumulating 3000 or less hours annually. Perfect for a trawler. 6BT's with a continuous rating are used where uninterrupted and unlimited service at full power is required. The serial number on a Cummins tells the story, a dealer can tell you anything you need to know about that engine based on that number.
Tony Athens is the go-to guy for Cummins Marine, he knows his Cummins line. Lots of info
on his website.
As far as reliability, the Cummins enjoys a reputation for longevity, repairable, and trouble-free if cared for. Mine is at 7000 hrs. uses no oil, no smoke, and efficient. It's a turbo, but has no seawater exchanger (keel cooled) and no aftercooler. Fully mechanical. Less is more.
If you want technical information on things diesel, check out boatdiesel.com.