I think Future Trunks was mostly the same since we last saw him personality-wise. In a way, he was more like a glorified, fanfic version of Future Trunks who was way more badass and skilled than he ever was in canon. He can now get epic rage boosts surpassing Gohan's, learn the Mafuba after watching a video, and unknowingly created his own variation of the Spirit Bomb. The manga hasn't included his RAGE SSJ form so hopefully it doesn't include the rest of these. It might be that Toei was worried that bringing him back wasn't fan service enough so they had to give him all these power-ups
.
(Hopefully I can add something worthy of what y'all have posted so far.
)
Gonna split this into
two three sections: Personality, Power (Mentality) and Power (Literal)
Personality: I honestly found it strange that during the android's attack, something that happened for 14+ years along with the death of his mentor and brother-like figure, Future Gohan, Trunks was (relatively) emotionally stable when he made it to the present. (English dub I don't count thanks to the famous "Trunks goes insane" meme.)
[Hell, he even let Cell try to kill him instead of just "running away" like he did in Super against Black.]
Yet, in DBSuper, he cried at the thought of Black terrorizing the world, the presumably killed Mai, and the death his mother, the last of which wasn't as evident as his grief over assumed dead Mai.
(I thought the casualty count was greater with the Androids than it was for Black? Maybe he's crying at the thought of his world never being able to find peace? I wouldn't, and don't want, to know what he's been going through.)
I would've liked/preferred if his character went in this direction. (Feel free to disagree.)
Trunks cries and grieves over Future Gohan's death and the human's deaths from the Androids > comes to the present > becomes an android killing machine, "hardened" heart in the process (like Vegeta said to Goku on Namek).
Years Later: > Fight Black > come to the present > be more levelheaded so that he can "think" back to when the Androids were the terrorist and think "how do I stop Black?" but with a touch of ptsd like he exhibited against Goku for mistaken identity.
tl;dr
Switch places for Trunks' badasserry from Z with Trunk's crying from Super and it'll look better to
The Retro Kakarotto , or in his opinion at least.
Power: Mentality) In Z, he desired to train with his father, the man he didn't know for 17/18 years but in Super, he initially just said "I'll let dad and Goku take care of Black." If Black is supposed to be the ultimate human murdering machine and Trunks was able to call on both of these Saiyan's help, you'd think he'd join the fray and ask to be a backup soldier or something against Black. Yet, all he did for "training" was spar with Vegeta against the Blue form and that was called off
Pretty early.
Literal) Now for the Super Saiyan Grade 3, Mafuba, Spirit Bomb-Sword part of the discussion.
-SSGrade 3 was something that Trunks (possibly Vegeta) achieved in the HTC after training in there for a year, and it showed Trunks as the novice fighter bc he didn't know about the Speed ∝ Power relationship. He faltered along his journey to grow stronger (with his dad).
-As for his
Legendary Super Saiyan
Demi God Rage form, till we get a better explanation, he just got it out of throwing a
very large hissy fit from hearing Zamasu and Black blaming him for the dead humans. I mean if it's extreme anger that was needed to trigger it, why didn't Gohan get this form against Cell, or Vegeta at Kakarot surpassing him, or Goku when Krillin died from Frieza?
-Mafuba... Okay I gotta ask, was Tien's attempt at the Mafuba against King Piccolo canon or filler? I don't remember my mangas. Cuz if it's canon, then a human just witnessed Roshi do the Mafuba one time, and he [Tien] just practiced several times, nobody bats an eye.
Trunks did it differently by witnessing it multiple times and practiced several times and shit goes wild with the fans.
I might be fine with putting this one off to the side.
-Spirit Bomb Sword. The only reason I think this could even
remotely be justified was cuz of how the Spirit bomb could be "reflected" if you have some good in you. [See: Frieza is still alive, theories says he's got a slim bit of good in him, and Kid Pure, manifestation of Evil, only one who actually died to the bomb.]
Merged Zamasu was nothing but selfish and (for lack of a better adjective) narrow-minded for his goals to purge the world of mortals. In the end, because of this purging, the spirit bomb deemed him evil. (And his immortal soul allowed him to live bodiless.)
But if we're not gonna look at this from the "Good vs Evil" angle for the spirit bomb but by the "pure power" angle, then I say Trunks learning it was bullshit.
Trunks didn't undergo the same training as Goku did. Subconsciously asking for Everyone's help is not enough to form the spirit bomb, let alone the very little population that is left from the underground bunker/Bulma's garage.
Even if someone brought up an argument about Krillin being able to learn the Spirit bomb he shared from Goku, that still failed as it wasn't enough to kill Vegeta and made sense it failed but Trunks' version was enough in power to defeat a merged Zamasu, a being > SSBlue Vegito, which doesn't make sense how it succeeded (in terms of power).
If anything, I think you're right @skar that Toei just gave Trunks these powers instead of Goku (or Vegeta) as more of fanservice.
tl;
really dr,
Z FTrunks made more sense than S FTrunks.
. 
I see what you did there.