I only have experience with three I-6 engines.
The 4.0L AMC/Jeep engines were legendary in their day, which I think is a testament to how bad engines were back in "the day". Build quality was wildly inconsistent, with some going 400,000 miles and others not making it out of warranty. Smooth is not a word you'd use with it. They were heavy, weak at around 190 HP/230 lb/ft toward the end of the run, had a relatively narrow power band, drank fuel, leaked oil like sieves, and had some issues that were absolutely maddening to diagnose like the misfire issues in the WJ Jeeps, the cracked heads, the crank position sensors that required patience and lots of beer to swap out, etc. I had two in my driveway and there'll never be another.
The 4.9L Ford was legendary also but unlike the Jeep engine I found that those were extremely reliable. I've seen those beat on like rented mules and they always came back for more. I put 30,000 miles on one hauling, towing, and commuting to college after my father had beaten it for 100,000 as his heavy equipment repair truck and it never had a single failure. I tended to even prefer that engine over the 5.0L V8 that was simultaneously offered. But, they too were a little rough around the edges and were very clearly meant to be a work tool.
The 5.9/6.7 Cummins is sort of in that rare class of legendary engines. Being a diesel it's obviously not smooth, nor quiet. It drinks a lot of fuel compared to the competitors' V8 offerings which seems odd, it's slow, and it tends to have its own personality. But, it'll last forever and parts are simple to find. A guy can keep those working for decades if he wants. But it has been in Rams for over 30 years and at some point you have to wonder if it'll be used another 30.
I suppose that none of these would stack up against a new, modern, clean sheet design I-6 in terms of efficiency, smoothness, and power per liter. But I think of them all as old engines that I grew up with that probably don't fit in or belong in today's world. If a new I-6 comes, I hope it is a very well designed and built engine and it feels like it belongs here.