sustainer doesnt sustain, it compressed, even out dynamic of your notes, to give impression of sustain. Hehe, blame it on boss for naming their pedal with that sustainer thingy and also the predecessor, roland sustanier, AS-1.
sustain, to a certain extend, depend on the material/construction of the geetar, setup(pickup, nuts, bridge, setting up of instrument blah blah blah etc) and volume of playing. Its made up of different factors, definitely not from a pedal. Effect pedal can only compliment what is already present in the instrument, and not do magic to add in much of whats not there in first place.
Any form of improvement of sound heard in an instrument when processed by any effect, is because of the effect processing and not the actual instrument improvement. Heh, its might seem minor, but theres a big different.
Anyway, above just imho. Important thign is to try as much as possible and find the best possible solution for the right context.