What are the main flaws you see in Cubarsi?
He is essentially a good footballer who is a mediocre defender.
Physically he is a bit problematic, he is niether tall (over 1.9m, or even on the average side like 1.86) or fast. Two essential traits for central defense.
At the moment he isn't strong either. This makes an ideal partner is someone who is both fast and big, which isn't fairly common.
His positioning and decision making are both very questionable, he is mistake prone, and once he is facing high level, he isn't an imposing CB.
Even his passing, while generally elite, he suffers under pressing, and tends to get rid of the ball to someone else rather than helping to get out of pressing (tbf, that would be too much to ask, but this makes his biggest skill not worth the trade off in big games).
Essentially he is like Lenglet without using his hand too much like the Clement used to.
And all that is fine for his age, but again age should be irrelevant if you are a starter.
I mean, for all the comparisons with PSG in order to make our players looks good and coach is the problem, the closest comparison to Cubarsi there is Beraldo.
A very talented footballer who PSG fans loved initially, but wasn't ready to be a good starting level CB at elite level. Lucho didn't give up on him, he didn't kick him out, but he brought two better CB and made him a rotation piece until he figures things out.
Right now, he is probably the 4th best CB in the team, with Eric surpassing him while Araujo and Christensen always being better. And even those aforementioned aren't worth a starting duo for Barca if we actually have a CL ambition.