Yeah Deductions are kind of pointless and it's odd that they're the only "ability" in the game and yet aren't used more than once a case. AA 5/6 had major issues with too many mini-games to the point where none ever got used enough to reach a sufficient and meaningful difficulty, but here it's this one thing but it's only ever used once per case so again there's still never enough usage to properly increase the difficulty. The entire first part is completely redundant and pointless and only really there for the amusingly bad and outlandish theories Holmes comes up with, but they're brain dead easy during review as all the framework has been established. It would be more tolerable and possibly more challenging/interesting a mode if we just had to figure stuff out from the get go and there were more red herrings for us to suss out rather than picking the one other thing in the scene that Holmes didn't see.
I guess you could say the multi-witness mechanic is another gameplay gimmick, but again it's not used enough and the way it is used is so spoon fed to you that it's really kind of pointless. That would be a perfect time to utilize something akin to Perceive, but instead we get a big sound effect, emoticon and special animation to tell us to question the other person.
The series has really struggled, and in my opinion failed, to improve and find a good alternative to Phoenix's Psyche-Lock ability as well as lots of evidence and presenting of it. Everything they've tried to introduce is too easy, too limited, too long or just not fun. Perceive was passable at best as it's close to Psyche Locks, but way too simplified and limited. Mood Matrix is beyond dumb, too situational, slow and just boringly simple/easy. While Divination Seance has all the same problems but to an even greater degree of being excruciatingly slow and limited in use. Deductions very much mirrors a lot of their problems. Edgeworth's Logic Chess from AAI was similarly underwhelming as it had a lot of the same aspects and intentions of Psyche-Locks but made the process needlessly convoluted and drawn out.
I wish they had used Logic from AAI and Thought Routes from AA5/6 for GAA. Logic would especially make sense in this context given the greater, thematic at least, focus on detective work. It would at least give the player some semblance of piecing things out themselves, but instead everything in spoonfed to us. Thought Routes is a fairly simple mechanic but it's fast and effective and could be build upon to increase the difficulty if it was utilized more. It could also be used effectively in conjunction with the Logic system. I also think it's especially welcome as the series seems to have abandoned high amounts of evidence and lightning rounds in cases where usually at the climax we are forced to quickly piece things together as we go and must present numerous pieces of evidence to support our claims in rapid fire succession that was really fun and exhilarating. So Thought Routes at least offers a streamlined, albeit much less challenging, version of that experience.