Most of German's "secret" weapons and projects were largely, if not completely, a waste of time. Of course, the same can be said of many Allied projects too. But, the difference is that the Allies, particularly the Western Allies, had the capacity and economy to support projects without dire impacts on war production in general.
To compound Germany's problems in this area, the Nazi system of individual fiefdoms that the various personalities surrounding Hitler had made often competed for resources and had parallel developments going that duplicated effort. Also, the political connections of the leadership in the government and business sectors often outweighed common sense or economic reality.
A list of truly idiotic and costly projects include:

The V-2. Rocketry on this scale should have remained nothing but a low priority program at most. This project was literally a crippling effect on Germany's economy. Reinvesting this program into a workable SAM might have been worthwhile

The X-4 AAM. Immature and impractical built to several thousand units before it was even successfully tested operationally.

Winterbaloon and Summerbaloon: These were speciality bombs for use in things like dam busting. They were marginally workable and likely would never see service as the bomber aircraft just were not available. Yet, hundreds of each were manufactured.

Hs 293 / Fritz X: Overmanufactured to the tune of thousands that would never be used. The program should have been sized to the potential aircraft available for their use. A few dozen a month would have been sufficent.

Me 163 A complete waste of resources.

Porsche Maus tank: A complete waste of resources.

Walther H2O2 boats. Should never have been proceeded with beyond basic experimental units.

Various midget subs and human torpedos. Desperation bred these disasters.

This is just the tip of a very large iceburg. The Germans had a penchant for designing these absurdities and then producing them without any valid data as to their usefulness. They also did not match requirements to capacity. Several of the above were built in larger numbers than could ever be used. This all made no sense whatsoever.