No ship is flawless.
The problem with NaLu is that Natsu is too dense to notice when Lucy is crushing on him, and Lucy is too shy to make a move except when she's drunk.
The problem with Gruvia is that both are trying too hard.
The problem with JerZa is that Jellal actively acts against his own best interest.
The problem with Gajevy is that Gajeel is too dense to notice he's about to become a father, and Levy is too shy to tell him until the baby pops out.
The problem with NaLi is that the author forgot about Lisanna.
The problem with GrayLu is that Lucy likes Natsu more than Gray, Gray likes Juvia more than Lucy, and Hiro Mashima likes NaLu and Gruvia more than GrayLu.
The problem with NatZa is that like 4 people ship it, and one of them is Chris-Tells-Tails.
The problem with GrayZa is that it has half as many fans as NatZa.
The problem with GrayTear is that the name more appropriately describes the fandom's salt at what happened to Ultear than the pairing.
The problem with Navia (NatsuXJuvia) is that Natsu likes Lucy more than Juvia, Juvia likes Gray more than Natsu, and Hiro Mashima doesn't know about this pairing.
The problem with NaKa (NatsuXTouka) is that there's no way a big boobed main character is losing to a flat chested villain.
The problem with LoLu is that there's no way the typical shounen protagonist is losing to a secondary playboy character.
The problem with GrayMi (GrayXMirajane) is that the one dude that shipped it has left us.
The problem with any ship involving Wendy is the FBI.