Cost is the main one.
Easy for someone not familar with the web and aquatic plants etc would have less trouble dosing etc I suppose, instructions for aquarium use are on the bottles labeled etc.
Some chelators for the cation NH4+, I do not think it'll really be that much difference in terms of plant growth though vs KNO3.
The KH2PO4 product is what is used in the SeaChem product + water etc.
You can go to the SeaChem site for more on their product line, they are about the best aquarium company around other than DIY folks and Greg's ferts etc.
If you are at this site already, there is little need to spend the extra $$$ for the Seachem stuff.
Order from Greg and for a few $$$, you will have a life time's supply.
Regards,
Tom Barr