I have no experience with SA mags. So I would not know if they are generally reliable or not. But here are a couple of things to consider:
First, the OEM magazine tube and the OEM magazine catch are two critically important parts to help ensure the OEM “feed reliability” that Glock is famous for. Why?
1) The magazine lips very much dictate the feed angle of the cartridges that are loaded inside the mag tube.
2) The mag catch and the notch on the mag tube determine exactly how and where the magazine is positioned inside the grip frame.
Obviously keeping these critical OEM parts intact guarantees that 1) and 2) above are also kept to Glock’s factory specs.
Second, even if you assume that the Z9 mags are all very reliable, limiting yourself to only use largely extended Z9 mags in the G43 all the time still does NOT make as much sense as the flush-fit S15 mags for the G43X, especially if you want to keep the grip length short for your CCW….
With the required steel mag catch, (while you are using the Z9 mags) you are very much giving up the ability to use the OEM flush-fit mags (with MagGuts+1) in your G43 and locking yourself into only using the extended Z9 mags, which would make the G43’s grip length as long as that of the G43X. This kills the sole advantage of the G43 - its superior concealability. Yet the Z9’ed G43 would still hold one less round than the stock G43X…. So you are better off just getting a stock G43X, than using the Z9 mags in your G43.