Those that say it is not reliable simply do not have the evidence to support their argument
As someone who supports the teaching of ded reckoning on the PPL syllabus, teaches these methods regularly at work, but also has a GPS which is used regularly at work, I think you're missing the point of the so-called "anti GPS" lobby.
I am not anti GPS. It is a wonderful tool, the best aid to navigation and situational awareness since, well, probably since the map.
However, it is vital to realise that
all navigation
must be based around flying the correct heading for the correct time. Then, a pilot must be aware that any planned heading and time will not be 100% accurate, and must have a means of adjusting a planned heading and time.
The method of adjusting the planned heading and time is what distinguishes one method of navigation from another. Looking out the window is not only a very good way of doing this, but it is also a very good way of teaching the principal.
Once the principal has been taught and understood, other methods can be introduced. This can include taking regular fixes from off-route navaids, or it can include tracking to or from a navaid, but in either case, it is suplimentary to steering a heading and measuring the time.
GPS is simply the next logical step in this process. Once again, the basic idea of steering a heading for a time is important, and then the GPS should be used to adjust the heading and time as necessary to arrive at the destination, or to confirm the current position along the planned track.
The danger of teaching GPS to PPLs is that, because of the presentation of data on the GPS, it is too easy to use it incorrectly, i.e. to fly by reference to the moving map, rather than by reference to compass and stopwatch. If students are allowed to do this during their training, they will most likely complete their training with no clear idea of how to navigate. If they have been taught to navigate properly, however, the use of GPS can be taught very easilly afterwards.
FFF
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PS - Dublinpilot, going back a few pages, but yes, I think we agree about the 72nm leg, except maybe for some semantics.