I was also reading about already Ukraine taking some of the 'relative' high ground in the region, which will cause the Russians even more difficulties.
They're rapidly attempting to build solid defences behind their lines, with the likely intention of bogging the UA down at some point if it gets that far.
The only other issue, however, is if Ukraine attempt to push too far, too quickly, and it allows the competent Russians units (as
@Eidean referred to) to isolate them.
A conventional defence against the advanced units, in open ground, could be something that the Ukrainians may struggle with, but it boils down to competence.