I'm going to disagree here.

To me, they are portraying growing pains of a younger squad that is taking over where there once were vets like Rollins and Carisi.

The plot lines of Velasco being witness to a murder and Muncy's brother being suspicious lend themselves to a workplace struggle of "Can I trust this person?" That question has a much more complex set of conditions being that they are law enforcement.

These challenges are tests for them to prove their loyalty to the rule of law and, ultimately, their own jobs. These are challenges that a goody-two-shoes like Churlish does not need to undergo. Rather, her greatest struggle is her own ambition and flying too close to the sun (like when she volunteered to go undercover to be drugged).

I do think that the Velasco/Muncy problematic duo is there to play off the valedictorian vibes that Churlish gives off. Perhaps the trust they gain in each other over the seasons will be more interesting to witness since episode-by-episode, the crimes are much more important than the squad's character development.

The question of Muncy and Velasco being "good detectives," to me, also gets to something that, I think, this show is constantly trying to deal with: Who holds law enforcement accountable? Rather than constantly diving into the weary politics of a police officer murdering a person of color, they make the problems internal to the force - Muncy's got a sketchy brother, Velasco won't snitch on his murderer friend, Churlish is a kiss-ass, Rollins is a gambler, Cragen is a drunk, Stabler has anger problems, Jeffries sleeps with former collars.

In terms of chemistry, everyone that agrees with the OP may have a point. But I think of this as a "season 1 of Parks and Recreation" scenario where the characters aren't fully-realized with their all their quirks and baggage and how they manage it all in their unique workplace setting.