My question is really about the importance of quality of piano keys in perspective of playing hardcore classical pieces (i.e.- gaspard de la nuit) like how much it matters - the touch sensitivity, feeling the vibrations through the keys, dynamics and articulation etc? Another way to understand the question is - can you play any advanced pieces (liszt/ravel/chopin etc) with same pleasure on a digital, and express it completely, or you would miss something? Or it is a matter of personal choice!
Once one's used a good quality grand piano, one realises that its action is as good as it gets, and anything less is, well, less.
Even most uprights seem lacking after, in some areas. Weighted actions will, however, vary, make/model to make/model. Grands will also have a far faster key recovery mechanism, which can help playing a fair bit. The sostenuto pedal found on grands is being used on digitals - possibly far easier to emulate electronically rather than physically on grands.
Fitting a weighted action, using hammers, is an option on keyboards which have to be put on stands, and makes them far heavier, so generally that idea comes in an upright piano-size casing - something OP doesn't want. There are, however, several makes and modles which do contain the equivalent of weighted action, although there are no strings for the hammers to hit, like on an acoustic. That 'feel' is improving all the time, but the better ones carry a bigger price tag. Even the feel of whatever material is used to surface the keys is a consideration.
But of course the weighted action, which helps determine the dynamics and articulation, is important. Were it not, grand pianos, even upright studios, wouldn't be used as often as they are!
All that said, there is a difference in the way different pianos play, and personal preferences will always surface. And, we can (and do) adapt our playing techniques to whatever we are playing at the time. And that includes use of pedals - subtly different on different pianos. Bit like driving a different car - the clutch bite point will differ, but we adapt.
So, the only real solution is to play as many different pianos/keyboards as possible in order to find one which suits you best. I could recommend two or three (but not on this site!) but all that would mean is I think they're really good. Maybe your opinion will differ.
Pianos did not did not drop finished from heaven, but were adapted over the centuries to cover additional requirements by pianists trying to get the most out them.
So the more similarity a digital instruments achieves, the better it will support serious interpretation.
Digital instruments covers a tremendous price range, and it is out of question, that quite a part of this range is hopelessly inadequate. I can't exclude, that if one plays exclusively on one of the upper-end range instruments, one might achieve an impressive level too, especially in non-professional context. But any competition, where you won't have the choice of the instrument, may pose a problem then.