His beat selection isn't the worst but a lot of them are just a few steps away from being good. Hip Hop is Dead has some decent beats, but coupled with him just rapping his song idea and his verse no one mastermind behind each track or trying to make a unique song holds it all back.
This beat isn't bad but it's average and repetitive, he's rapping too much. You heard the first 10 seconds of the song you heard the whole song. bassline NEVER changes. No one is going back to listen to that except his hardcore fans.
Then he tries something new with this Detective Bullshyt song and no producer got him to reign it in. The lyrics hardly match the pace of the beat or the mood. Nas is a great writer but he needed a great producer to help him.
Check out Black Republican.
Simple song structure, but the beat has legs, not just 4 bars and repeats. Obviously Jay has an ear for this type of stuff his flow and emotion matches the song. Nas raps for only 40 seconds and just spits. It may be a Nas track featuring Jay on a Nas album but that's a Jay-Z song.
Compare that to Kendrick where his beats have a variety his flow is varied so it's not just him spitting his verse. He puts emotion behind it, his verse and production match each other. This isn't a mistake or a random event it took work. From the intro it creates a long structure, the beat doesn't switch up too much but the bassline has variety, beat drops and voice flow give it variety.
Nas isn't alone in this. Check a lot of rappers from 97-2008. A beat and a few bars, that's all they had. That shyt won't get play now.
When people say Nas has trash beats, it's a simplified way of saying his songs are repetitive and terrible production.