Hmm, it's been a while since I've done this.
1. Jinbe's strength. While I think Luffy has much more potential than Jinbei regarding strength, it's never shown that Jinbe is a notch below Luffy. Also take into account that Ace and Jinbe came to a draw when they fought. They always meniton Ace being stronger than Luffy throughout the story, which they also emphasized in the flashback, so if Jinbe and Ace both drawed, what does that say about Luffy? Only that Jinbei, if he were to join, may possibly be stronger than the captain.
It's not that Jinbe would be considered weak. It's more that Luffy/Zoro/Sanji are just that strong. Sanji already fought alongside him in Fishman Island and they seemed pretty comparable at the time.
Luffy was inferior to Ace, sure, but that was before he had a two year timeskip for focused training and with an age disadvantage. There's no reason to think he's still weaker than Ace was. The fight between Ace and Jinbe also occurred when Ace was a rookie. As in, the same age Luffy was before the timeskip. 19 year old Luffy being weaker than 17 year old Ace would just be wrong.
Back during Water 7, Franky appeared to be on par with Luffy. But one scene with the two of them and Lucci in Enies Lobby shattered that concept, and it didn't really compromise Franky in the process. Then we got the Ryuuma fight with an amazed Franky as a witness which solidified just how strong Zoro was as well. Oda's good at doing this.
2. His bounty is too high. Luffy and Jinbe both have bounties around 400 million. There needs to be some spacing here like what Luffy and Zoro have at the very least. Their bounties may distance themselves in the future, but considering Jinbe's capabilities, I don't see a wide gap coming unless they suddenly make Jinbe less capable.
Luffy's bounty only surpassed Robin's a couple chapters before she joined. Even then, her bounty was still higher than Zoro's for multiple arcs. It didn't matter from a reader's standpoint at all because Crocodile trounced Robin before Luffy defeated him. We're talking about hundreds of chapters with Luffy at 100, Robin at 79, and Zoro at 60. And hell, Sanji's bounty is still lower than Robin's to this day.
Again, it's not that Jinbe has to be made weak, but rather that Luffy (and you could include Zoro and Sanji) just has to be that strong. It's not like that's some unreasonable feat.
3. Too much of a mentor role. I don't like the idea of Jinbe giving Luffy advice on things after the time skip. To me, it seems like it would take away what experience Luffy has learned after the war. And we already have Zoro to keep Luffy in check.
Various crew members are giving Luffy advice all the time. A lot of the time he just ignores them.
4. I don't know about this one, but his decision not to join right away is suspicious. Not sure what Oda has planned here, but I think Jinbe may end up staying a Straw Hat ally rather than a full-fledge member.
A character Luffy pushes for that promises to join, who doesn't fulfill his promise? I dunno… That'd be sort of like Brook not joining after he got his shadow back, but the promise would make it even more severe.
). I'm not worried on that account. At worst, he'll get the Robin treatment and just not have any fights that don't end in him utterly annihilating the opponent. Personality-wise, before Fishman Island I would have said he wouldn't have fit in. After FI, I could go either way. He hasn't clicked with me as a character yet, but if he shows up again in a few arcs, he just might.
