FES has chosen there system on several aspects. One is that they want to be a competitors to combustion engines in new gliders. Therefor is the extra payload in the fuselage not an issue for them. But for me who wants to retrofit. The Mass and Balance can easily be solved by an extra weight at the fin.

When it comes to lifetime of batteries is my experience that it is severely dependent on the usage and abusage.
Best cast scenarios will they hold up for about 1000 charging cycles, maybe more with good battery monitoring.
What kills them is abuse.
Deep discharging is the main problem, do not discharge your lithiumcells to deep (this also goes for you cellphone or laptop), unless necessary to save your life.
Next is heat, do not let them be hot during charging or discharging.
Third is to discharge them to fast i.e. overload by taking out to much power, this generates heat and is not good. The last is to store the batteries discharged to long time, all batteries have a self discharge and long storage without proper storage charge will put them into deep discharge mode.
I expect the designers of the electrical systems to consider this aspects and therefor put in battery monitoring system that keeps them in charge, in correct temperature and not allow the user to deep discharge them. I would expect them to have a long life time, making 30-50 starts per year give me more than 20 years of usage. This is more than I can foreseen as a glider owner, after 20 year will new batteries be better and the cost be motivated in extra performance compared to just replacing them.