The only major issue to look out for in the 4.2L I6 is a situation where the intake valve seats are "soft" and cause low compression and a miss at idle. It won't come up straight away on a search, but if you look hard enough, you'll find it.
Ultimately, GM made a lot of these engines and the valve seat issue isn't really a big one. Pretty much, if you've made it to 75,000 miles, you're in the clear as the valve seat issue should have made itself known by then.
That said, I own a 2006 Trailblazer with just over 100K. It's been perfect. The only thing that could make it better would be coupling it to a 6L80 trans. I think the extra two gears would really help with keeping the engine in the upper revs when necessary and, just maybe, add a couple MPG to the fuel economy numbers.
Unfortunately, a long and tall I6 doesn't make packaging easy. While the 4.2L did spawn I5 and I4 variants, ultimately, the platform must be designed around the engine. Unless you're committed to the I6, like BMW and Volvo, this is difficult and costly. GM gave a good effort and came out with a good engine, but I'd imagine that adapting it to other platforms was just too costly and with customer favor trending away from truck-based SUVs and more toward crossovers, the decision to scrap any further development on Trailblazer platform, and it's I6 engine, wasn't a hard one.