I'd assume that since the power they get is what Shurimans consider "divine" that it works in ways said divine powers determine to operate best. The powers likely relate to the person in question, what they do, what they've done and what they intend to do. Renekton might seem like he got the short end of the stick but he's likely far more physically capable than any of the other Ascended we know of, even among their standards.
Let's also not forget that Renekton is also insane, and any special powers he might've got in line with those of his brother might've either been forgotten/inaccessible due to insanity, or he's too enraged to even remember/be arsed to use them.
Nasus and Renekton almost certainly ascended at the same time, so potentially the gods or whatever divinity means for Shurimans had different plans for the two. Imagine you're a Shuriman god when there's a void invasion/civil war/Brackern invasion/Freljord invasion/whatever. Two brothers come up and are like "Hey we're a curator and a warrior, please grant us your boon so we may protect our home!" and you're like "Well shit time to get to work." You look at the two and say "Well one's a warrior, so best to embelish that and let him fight like a monster. The other's a curator, so let's embellish that so he can help the people at home while the other goes out and kills shit." Hell it'd also explain atleast with a bit of stretching why in the mural of a Xer'Sai vs Renekton in the Rek'Sai journal that only Renekton appeared; Renekton was the one on the front lines while Nasus kept the populace safe and calm. Would make for an interesting point for an insane Renekton to bring up as well during their encounter.
: "Brother, I love you, I always have! We are family, do you not see?"
: "Then why was it brother that it was I who toiled and killed for Shurima?! Why was it that I was there at the forefront while you cowered from the fight?!"
Also a pain point for Renekton to help explain how Xerath drove him mad over the 1,000 years they spent imprisoned together.
I ended up going more on a tangent about Renekton himself than whether Ascended are all made equal but I'd say they're made on a case by case basis depending on a bunch of circumstances. I mean hell Azir's powers are also likely affected by the fact he was brought back from the dead before being Ascended. I feel like we'd have more details to go on if we actually got another Ascended in to the story somehow, I mean half of the Ascended we know of are from the very last part of Shurima's life, and the other two are super ancient. Surely there must've been some others and surely some of those other Ascended have gotta be alive, right? They're immortal demi-gods!
: His Ascension restored Shurima's capital to its previous glory, has sand magic that allows him to create many things from recreating moments in time to legions of sand soldiers.
His Ascension destroyed Shurima's capital,in his own words the only "true" ascended.
Being of pure energy, able to use "pure" magic and is able to "see the forces that hold the universe together."
