Okay, first, Decidueye is not bad. I used it and it was awesome. Just some FYI, at the end I sum it up, this chunky text is just explaining when they're useful. Anyway, I'm no Meta Player, but here are the things I can see:
Decidueye is a Grass/Ghost with Overgrow and Long Reach as a hidden ability, which can be useful in some cases. Decidueye learns Normal, Grass, Ghost, Flying, Bug and Dark moves without TM/HMs. Decidueye's best stat is Atk, then Sp.Atk/Sp.Def are tied.
Incineroar is a Fire/Dark with Blaze and Intimidate as a hidden ability. Intimidate is broken, so if you're willing to grind for a hidden ability, go ahead. Incineroar learns Fighting, Dark, Fire, Normal, Ghost and Dragon moves without TM/HMs. Incineroar's best stat is Atk, then Hp.
Primarina is a Water/Fairy with Torrent and Liquid Voice as a hidden ability. Liquid Voice, as far as I can tell, only affects a few of her non-TM/HM moves, so Torrent is probably better. Primarina learns Fairy, Normal, Water and Ice moves without TM/HMs. Primarina's best stat is Sp.Atk, then Sp.Def.
Going by the Totems and Elite Four, Incineroar destroys your Ultra Moon Totem Alolan Raticate (I played US) with his 4X powered Fighting moves, closely followed by Primarina with her 2X powered Fairy moves. Decidueye remains obsolete. However, oddly enough, only Decidueye has a chance at your Totem Araquanid, with his 2X powered Flying moves. Incineroar's Fire moves and Primarina's Water AND Ice moves are nothing against that thing, plus Incineroar's actual WEAKNESS to it. The next, Salazzle, is obliterating Decidueye, having a 50-50 shot at Incineroar, and is being obliterated by Primarina's Water moves. The next, Togedumaru, won't even batt an eye at Decidueye's attacks, takes Primarina's normally, and succumbs to Incineroar's wrath. The next, Mimikyu, has neutral damage from everyone but Decidueye, who destroys it with its signature (not to mention STAB) Ghost attacks. Incineroar's Fighting moves do nothing at all, but that's hardly a problem. The last Totem, Kommo-o, puts shame to Incineroar's name, with both of Incineroar's STAB attacks dealing half. However, Decidueye's immunity AND supereffective Flying and Primarina's 4X effective Fairy moves melt - and I mean MELT - this Totem. The first Elite Four member uses Steel Types, so a clear point loss for Primarina and a point gain for Incineroar. The second uses Rock, for which all starters have supereffective moves. For the Ghost member, only Primarina doesn't have any superffectivness. And finally, the Flying member, for which Decidueye and Incineroar suck and Primarina's single Ice attack is useful. As for Ultra Necrozma, they all beat it. This isn't cool, or unique, though - I one-shot it with a Malamar (One of its effects is INSANE) using a Dark attack. Factoring in the fact that they all have the same stat total, these are the rankings:
In conclusion: Primarina is better statistically. That means that Primarina could also suck, and even then I didn't factor in natures, IVs, the TM/HMs and a bunch of other statistics, but quantifying them is next to impossible, so yeah. But still, there are moments when your starter will struggle, and times when they will shine, so you need to remember that you have other team members. I'd also suggest playing through the game with every starter, just to get a different feel. I haven't - too busy playing Blue - but at some point in time I would very much like too.
So, um, yeah. That's all.