I had issues using Utilt like a few months ago, then I learned that it was a disjoint(the whole thing. Tongue = No hurtbox) and since then it's been a lot easier to use, it's also godly as it kill confirms into Uair at high %s and it's one of your best non footstool combo starters. You can Nair, Perfect Pivot Utilt(tiny window) into Uair Spike, into Uair, into Uair spike until like 80% where you can Uair spike, Dtilt Usmash. This requires good reaction or reads for DI. Getting an Utilt can be huge against light characters on non-high ceiling stages. I've killed fox at like 80 with rage from Utilt to Uair from the ground.
Dtilt is a good grounded poke. Safe when properly spaced, at low %s you can grab after it, at mid percents you can Dtilt into Jab and sometimes Dtilt Perfect Pivot Utilt(so you can literally combo people from like 40 to 80 from 1 dtilt and good reaction and tech). From around 60% you can Dtilt Usmash(you need to Jump Cancel USmash I do believe, gives you a little more slide). then at around 120% you can get Dtilt to Fair which sometimes True combos people but for the most part is a 50-50, you can also do Dtilt into Uair spike and then just Dsmash/Fsmash them at around the same %.
It's also pretty good in the neutral IMO, not great like Jab 1, Fair and water shuriken which imo are greninja's best neutral tools but Dtilt is quick, converts nicely and people sometimes don't expect it. If you see your opponent approaching you through the air, sometimes you can, instead of Utilt, Perfect Pivot away Dtilt to cover the ground they'll land on.
Also anyone who's trying to main Greninja should watch this video:
https://www.youtube.com/watch?v=Z78INd80SNo
it has some really fancy stuff (mostly footstool combos) but it also have pretty much all the B&B combos. The only one that I feel like could use an update is SHFF Nair - Dash Attack - Fair, it got even better with the new patch, so it's definitely still great but I feel like SHFF Nair --> Dtilt --> Grab (I think you can squeeze 1 pummel but I'm not sure))--> Uthrow --> Uair might be better: A) it deals a little less damage B) you can try to Uair spike at low %s and get huge conversions that would mean more % dealt. C) it leaves the opponent right above you --> you can catch their landing.