First I have to say that I was never involved in the Tanto project since it was before my time in the AkaModell. But I spoke to some of the people who were involved, so I might still be able to give you an adequate answer.
There were multiple reasons why the Tanto-project was never as succesfull as intended.
The first problem were the batteries itself, which were located inside the wing. Due to the very long battery cables, there was a lot of resistance and powerloss on the way towards the speed controller. Additionally, the batteries expanded while being under heavy load which led to a deformation of the wing surface.
Coming to the second problem: the wing itself. Since the wing airfoil was designed as a laminar flow airfoil, the surface quality plays an important role. As already mentioned, because of the expanding batteries the surface was already deforming which led to more aerodynamic drag. Additionally, the wings were built out of negative molds which were milled in Ureol. The material which was used had not visible porosity in its surface. As a consequence the built wings had a rough surface which disturbed the laminar flow and increased drag.
Because of the increased drag and the resistance losses in the cables the optimum flightspeed couldn‘t be reached.
All these problems could be solved with existing technology: speed lipo-batteries are available now (which can take much higher currents) and the wing could be painted with clear cote and polished.
Since the Tanto was designed for the F3S rules of 2014 (as far as i rember correctly), it is also questionale wether the airplane would still fit the changed rules and wether its design point is still suitable.
Sadly the motivation of the team decreased after the failed atempts to reach 500 km/h and the project is on a hold since 2015.
Furthermore, our pilots who are capable of flying an airplane over 500 km/h have either quit their active time at the AkaModell or have very limited time because of their jobs or ongoing studies.
I hope this answers your question. If you have more questions about the project feel free to ask or PM me.
Greetings
Maxi