Short answer: use something else
Some semi-technical details follow (so ignore if not interested).
The MSDS for
Sustain® Summer Shield says it is salt plus "Sustain Shield (proprietary)". The byproducts of decomposition/combustion and the oral toxicity to rats are somewhat similar to that of sodium cyanurate as with Instant Pool Water Conditioner (liquid CYA), though this could be a related compound such as glycouril instead. Basically, whatever this is sounds very similar to CYA in that it holds a "reserve" of chlorine releasing it as needed. Also, with any chlorine, you'd need something to protect it from breakdown in sunlight. PPG says in
this link that Sustain Shield is protected by patent which I believe is
this patent where there is essentially a CYA-like substance that binds with chlorine to form a reserve (the chemical is a derivative of
2-oxazolidinone and combines with chlorine to form a derivative of 3-chloro-2-oxazolidinone -- a paper describing the bactericidal properties of such a compound is
here).
Sustain® Shield Energizer Tablets are Cal-Hypo tablets (> 65%).
Sustain Chlorinating (blue) Tablets are also Cal-Hypo (> 65%).
With Cal-Hypo, for every 10 ppm Free Chlorine (FC), it also increases Calcium Hardness (CH) by 7 ppm so you need significant water dilution to keep CH in check, though it's easier to do than keeping CYA in check from Trichlor tabs.
Bottom line, this system is far more expensive than using a regular chlorine source. As for the algae-prevention properties, one can use PolyQuat 60 weekly if they believe they will be unable to properly maintain a chlorine level during the week, though this isn't necessary if you simply maintain an appropriate chlorine level. In fact, even the
PPG guarantee for Sustain is void if you don't maintain 1-3 ppm FC and 1.5-5 ppm Shield concentrations. Note that stabilizer (CYA) is used separately in the system from "Shield" and is supposed to be at 30-50 ppm.
I maintain my 16,000 gallon pool shown here and here using 12.5% chlorinating liquid twice a week (I have a pool cover) plus a small amount of acid every month or two. It costs me around $17 per month in chemicals (at a daily FC usage of a little under 1 ppm).