I have seen the Elicker, the Koellner units here in the U.S. On the smaller size seglers under 4 meters there seems to be no problems to speak of.
When you advance to 12-18 kilo seglers, then the problems need to be carefully managed. they are heat build up in the motor and batteies.
I have tested a large TM430-30 10pole/29w Torcman, the Kohler Actro 32/5. Jochen Zaiser has worked closely with me to design a custom motor to meet our requirements.
Using the above Torcman on a pylon mount, with a 20/14 RF folding prop (for testing only), 28 nimh 2600 mah cells, we produced 17 pounds (7.7 kilo)static thrust. We used 35-45 amps draw. The rpm's were about 4500-5000, if I remember correctly.
That translates into about 50 watts per pound or 23 watts per kilo if my math is correct. This is more than enough to launch a 14-16 kilo sailplane.
There are systems using less power on the market. I can not speak for them, but I have heard of heating problems with motors, possibly due to a higher amp draw? I do not know if this is because the motors are run for too long a time or what.
The Lehner motors have the best reputation in the Elicker and Koellner units because they tend to run a little cooler I am told.
In our testing with the Torcman, the motor ran full throttle on the bench for three minutes and did not overheat a great amount. (we did use some cooling fans on the batteries) The batteries only got warm to the touch. I think the important thing is to keep the amp draw low as possible in the given (watt) output range.
I have conducted some tests with 3300 pushed cells and they do perform a little better.
I would be curious to the experience in Germany with commercially available units.
JD