Whether vegetation is good or bad depends on the angles, soil type, soil moisture regime and the type of vegetation it is not a black or white subject.
Also what happens after a major intervention like felling trees. If the trees had become dense, there was probably little vegetation under them, so the earthwork is initially left close to bare.
If soil fertility is high (lots of chipped material left to rot) then round here cuttings seem to go to bramble or ivy, which are very different to a permanent grass cover (with a dense thatch of grass at the bottom).
The grassy embankments seen before 1970 only stayed that way because they were cut, and just as much because the cut material was taken off, which held down soil fertility. Although the above ground vegetation was cut off, the roots were not killed so remained alive. Chopping down trees and poisioning the stumps is a very different effect, as others have said the roots die, but also the fertility is also released by chipping the wood over the line side to rot.
Unfortunately, appreciation of the role of soil fertility is poor even within the landscape profession, and much worse in either civil engineering or network rail (some of the weird policies adopted by Network Rail demonstrate this).
