This is not something I know much about, but one comment does occur...
...the effective range of my DECT phones is much greater than my WiFi. They can be used all the way down my garden and beyond, whereas WiFi peters out about half the distance.
On the same note, one big problem with everything WiFi is that the frequencies are shared with all manner of other legitimate devices, from baby alarms to radio controlled toys, garage door openers, security cameras and so on. They are all quite legal but IMHO can conspire to drastically reduce WiFi reliability. I wonder if DECT enjoys better protection?