So this effectively makes the chlorine level 2-3 ppm FC higher at the heater than it otherwise would be. With CYA in the water, the active chlorine level will still be low but perhaps would be equivalent to 0.14 ppm FC with no CYA instead of 0.08 ppm FC with no CYA. The reason Trichlor pucks in the skimmer are a problem is that when the pump is off they build up high chlorine AND low pH which then gets blasted to the heater on every pump cycle. Low pH is far more harmful to heaters than chlorine unless the FC level is very high and there is no CYA (i.e. a very high active chlorine level).
Now an SWCG cell may output only somewhat higher FC since it's on-time is probably not 100% (but 50% on-time would be double the FC compared to The Liquidator, all else equal), but the reason it is put after the heater isn't only because of this higher FC level but that part of the water from the cell is very acidic while other parts are very basic. So unless the water coming out of the cell was thoroughly mixed together, one part of the stream will be very acidic and at a high chlorine level which is a bad combo. By the time such water comes out of the returns, it gets mixed with the bulk water so is not a problem.
Let's say the pH were maintained at 7.5 at all times for 100% of the water.
How much Active Chlorine would it take to damage the heater?