#2 His limited skillset
Without a doubt, Adrien Rabiot is a good passer of the ball, with high completion rates, but most of his passes are usually short, diagonal and less risky passes that prioritize safety first other than offer any attacking threat.
He predominantly stays behind in PSG's attacking forays, sitting back while the likes of Veratti, Neymar, Mbappe and Cavani surge forward, evidenced by his low returns on goals (he has scored just 22 goals in 208 appearances across all competitions for PSG in seven seasons).
As Sergio Busquets has shown all through his career, a deep-lying defensive midfielder not posting high returns on goals is not too much of an issue, but what the World Cup winner boasts over Rabiot is his innate ability to dictate play and build-up play from the middle third and create goalscoring chances.
Rabiot if purchased will primarily be deployed as a holding midfielder, and it will be a near impossible task for him to displace Sergio Busquets, while Arthur's multi-functionality means he will also be referred to the one-dimensional Rabiot in Busquet's stead.
In other areas of midfield, Rabiot's lack of pace or ability to create space for himself means that he would still be behind the likes of Arturo Vidal, Rakitic, Malcom and Philippe Coutinho.
Rabiot is likely to command a fairly high transfer fee despite having just one year left on his contract (as Ligue 1 players don't have a buyout clause due to LFP rules) if purchased by Barcelona, and expending such amount on someone who doesn't bring anything different to Barcelona's team could be an exercise in futility.