Back during my (HO) steam-dabbling days, I PREFERRED plastic steam engines over die cast. A major part of the biggest enjoyment I received from my steam dabble days was kit-bashing steam. That was much more readily and easily accomplished on a plastic shell than a die cast shell.
So IF I was still in S scale, I would have zero issues with a well done steam engine with a plastic boiler/superstructure PROVIDED it could be had/easily adapted to scale.
Unfortunately, this thread is a good example of the many issues that exist in S scale, among them, the part where there seems to be several niches trying to get their way to be accepted as the best way. Adaptability of product would SEEM to be the way a mfg'er ought to perceive being a good way to proceed, but apparently it is not.