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.
I'd get KNO3 and KH2PO4 at the bare minimum. Tom will probably recommend Tropica Master Grow (or whatever it's called now) for the Micro source. Otherwise, CSM+B will work and so would Seachem Flourish. I really don't know that K2SO4 is going to be needed.
You can get the CMS+B if you want.
I like TMG personally.
The GH booster has a fair amount of K2SO4, so that, Ca, SO4, Mg need not added if you add about 1-2 degrees of extra GH per week.
The added GH will not do any harm to any plant/fish etc.
Not enough will do harm.
The GH Booster won't be congealed into one solid lump in a jar, and consequently it mixes in a bit better ! LOL.
There's no denying the overall quality of Seachem products. It's up to the individual to decide where, and what they want to spend their money on. Greg does it as a service to the community. There sure isn't any profit in it.