I feel as though that they intentionally avoided making a heavty amount of changes to most characters (barring Luigi and Samus), because they wanted to test and see how the new shieldstun mechanics change various styles of play. They probably know just how important a global change such as this one can be, so I am thinking that they are waiting to see how everything plays out before going back to making readjustments to the rest of the cast.
Luigi's grab game was changed to compensate for this, as it was the most potent in the game. Meaning that they plan on keeping the new shieldstun mechanics in the long run. Because since it'll probably be easier to land a grab with Luigi now (Fireball and falling aerials), they thought that it might've been a fair trade to exhange his extremely potent grab combos with the increased chance of landing a grab in the first place. To me, the intentions of this trade are very clear, as otherwise they would've edited other values to reduce Luigi's ability to land grabs, such as they did before with Fireballs (which didn't get changed this time around). Overall, the reason why they focused on Luigi in particular is because he is a character that is extremely reliant on his grabs, more-so than any other character. Meaning that they had to change him to go along with the new shieldstun mechanics. They even gave him additional buffs in his Dair and Dsmash to help compensate for his losses.
Samus, on the other hand, was simply buffed because she is commonly seen as being bad. Likewise for the Nair buffs given to Ganondorf and Bowser. And most Custom Moves that were buffed were seen as being bad anyways.
No idea about the Luma changes, but lol
Anyways, I am not too worried about this. They are probably saving the bigger patch changes for the future, such as when they start releasing new stages/DLC, or even the Ballot Characters. But those might take awhile anyways. Personally, I'm just going to give it time. I always do anyways, but regardless~