If you live or drive at higher elevation, like I do, the I6 turbo provides a lot more power. At my 6300' base elevation, my 357hp V8 only makes around 292 hp. I still only get 15mpg over the last 7k miles.
The inline 6 should be very smooth because inline 6 configuration is inherently balanced (as is a V12), and it will still produce the same peak 425 hp as at sea level. Compared to the V8, I should have better fuel economy (closer to the current V6 in combined driving) while having an extra 133hp, which is a HUGE power gain (and even more torque, at lower RPMs). So I will go from mediocre power Hemi to a very fast SUV, as I drive anywhere up to 10k ft above sea level, which will make a HUGE difference.
Even at sea level, the I6 turbo will have nearly 75 extra HP and a lot more torque, all from a more fuel efficient engine than the current Hemi.
Turbos notoriously have a hard time meeting their EPA numbers vs real world testing (Car and Driver does a good 75mph highway test that provides more accurate expectations than the EPA), because those tests are based on light throttle usage. smaller turbo motors have less parasitic loss at light throttle b/c the smaller motors have less drag and parasitic loss than larger displacement motors), but it still takes roughly the same amount of fuel to make big horsepower, so if you have a heavy foot, you burn more fuel anyway (ie to make 425 hp in the I6 vs 5.7L V8 357hp.)
That all said, the older motors will likely be more reliable in the early years of the I6 turbo both because the older motors have been around forever, and turbo motors, which have become pretty highly reliable as a class these days, are more complex, run at much higher compression, and this is a new product. That said, I have a 3.0L I6 twin turbo in my wife's 2008 BMW 535xi wagon, and at 160k miles, at 14 years old, the engine (and transmission) are still as quiet, powerful and responsive as when it rolled off the assembly line. The car has never been chipped or modified, but it did have new turbos around 40k miles as there was an engineering problem with the original ones, and that engine was a pretty new design at the time.
What can we expect from Stellantis re reliability? Well, the design has been in use by BMW for 15+ years, and Stellantis has been working on designing a twin turbo 6 for about that long. From what I hear, they were originally designing a TT V6 starting around 15 years ago, but switched to an I6 configuration later in the process (around 5 years ago or so IIRC). They have developed a 525 hp "Hurricane" version that's going in the Grand Wagoneer right now, and will have a 425hp version for the Grand Cherokee (sounds like not until '24 MY). \
They're pumping a lot of boost into a 3.0L motor, but have spent a lot of time testing it, so it should be pretty good, but I wouldn't be surprised if there are some early teething issues.
As someone who has had some highly modified cars (including an 800hp Audi alllroad with a custom 3.0L V6), I REALLY want the new I6 TT motor in my car, but if you value reliability above all else, you might wait a few years before buying one. For my use case, at high elevation, an extra 133 hp and better fuel economy would be a godsend. My Jeep dealer is 3 miles away, and I have two other cars, so I'm willing to risk being an early adopter, knowing I'll have an 8/125k warranty, too.