Being corrosive to copper is kind of a big deal.
initially, i was only thinking about the blocks - which is fairly easily remedied by getting nickel plated blocks (it seems everything thing these days has a nickel plated option). but what i hadn't thought about at the time is the rest of the system. specifically, radiators - even though they wouldn't necessarily be used in a TEC chilled system. /shrug i don't know - if there is any other component other than blocks which are not readily nickel plated then that would obviosuly be a show-stopper. **compressor type system would obviously be a no-go, but since this is posted in the TEC section.....

...but ~15% lower thermal conductance than water at 32C.
ahhh - for the life of me, i could not find any thermal conductance info on aqua ammonia lol....i kind'a just assumed similar (uniform) properties compared to H2O. but something also to consider when comparing to eg/h2o - the common mixture with eth glycol is 40/60 or 50/50 (in order to get to a sufficient tolerance on the low end at around -38C) - and with this mixture you need +50 to 100% flow at 0-15C for equivalent perameters. with aqua ammonia (20-33%) you will likely have much better viscosity/flow at those temps. my comparison was basically thus:

Aqua Ammonia (compared to eth glycol): better viscosity than eg due to lower percentage solution, better than eg heat capacity (on par with h2o), same (lower w/respect to h2o) thermal conductance as eg, HIGHLY corrosive to Cu, Ag, Al, et al...