It depends on the quality of the instrument.
I currently practise on a Kawai CA79 which has a very nice action. It's not exactly the same as a grand, but it feels good. It's way better to practise on than the shoddy old upright we had before I bought the digital.
So the relationship
Great digital > shoddy upright
is at least true.
I'd also say, considering some grands I've played on, that
Great digital > shoddy grand
However, comparing a great grand to a great digital, I'd say
great (or perhaps even merely good) grand > great digital
because currently, an aucoustic instrument is simply more the "real deal" than an attempt at making a simulation of the "real deal", which is what digital instruments do
Bad uprights can be astonishingly terrible, and bad digitals do just not feel any way close to how it feels to play a grand, so they are not good to habitually practise on if you are preparing virtuoso repertoire for a grand.