Um, I don't think that Naganadel will be on equal footing with the other UB's, I bet it'll be a tad weaker compared to the other UB's, but still definitely would be competitive as after all it's still an UB. By Hala's statement in SM51 it's clear that Tapus = UBs, and considering how powerful Tapu Koko was depicted, the Tapus and hence UBs are quite strong. Yeah Naganadel very possibly would be depicted weaker than the other UBs considering its lesser base stat total in games + it's a captured Pokemon which Ash very likely will use in competitive battling and it would likely be his non-ace, but I highly doubt that it'll be so much weaker than the other UBs that it can't even be competitive with them.
Yeah it's fascination with light could possibly influence on the arc climax, forming some sort of a connection with Ultra Necrozma or so, but I still doubt that it'd play it's role completely in the Meloetta way, without showing any strength or so, because:
Meloetta is a cute, sweet looking Pokemon, not really a Pokemon which looks much of a battler and Poipole won't be that kind of Pokemon once it evolves. Naganadel is a huge beast like creature, so it'd be natural if the writers have it show its strength in the climax arc as well and not only indirectly affect the outcome of the arc like Meloetta.