In most cases where I have come across the continuous descent approach, a DME is provided.
Thus while not capturing the glideslope from below does leave the posibility of establishing on a false glideslope, SOPs which require crosschecking of height versus distance on a regular basis and crosschecking rate of descent required against groundspeed can ensure that the correct glideslope is being followed.....regardless of how it is captured.
Regards,
DFC