I certainly understand the appeal of plastic weapons from both cost, weight and care perspectives... But eventually plastics will fail... All materials degrade. Plastics and plastic polymers just do so in a way that's often not visible until it fails -- and they're constantly being degraded by stuff like ozone that there's no way to protect them against. With metals, I feel like you can 1) see the corrosion and 2) do a lot more in preventative maintenance to protect your gun... Stuff like the beaver barf -- IMO a big part of its durability is due to the wood chips, today there's fiber glass impregnated that does similar. So I certainly get it...
With a gun the HK91, it'd be well served with a polymer outer receiver to protect the easily dented operating channel... Take a look at the AR18 upper receiver steel vs many of these new polymer guns. The amount of metal inside the polymer guns along the operating channel is definitely less thick than the AR18... Point being, stamped receiver guns do stand to benefit from polymer outers -- allowing them to be built to tighter tolerances b/c a small dent won't put them out of commission since a small dent is now much less likely to occur than w/ just bare metal...
But then even more extreme you have guns like the G36 that have a polymer encased trunnion, which has led to German soldiers saying its lack of accuracy when hot makes the gun combat ineffective...
So my polymer hesitations are very much related to how used. But in most cases, I see very little drawback from adding steel reinforcement at critical stress points... Especially when recognizing that when very hot or very cold, polymer becomes more soft or brittle than metal at the same temps... So it's less durable from that perspective as well. And for whatever reason polymers seem to be less able to take certain forms of stress too -- when made for strength they're more brittle and prone to brakes, like shearing mag lugs on polymer mags. Adding steel there adds little weight and a lot of added durability, wear resistance and peace of mind...
Bottom line, for me, I buy weapons for life and don't carry them for a living so the slight added weight and slight added costs are negligible for me. Especially for rifles. Handguns, especially CC, I'm less concerned about polymer. In part, I know its due to lower costs of polymer handguns and less stress the cartridges/recoil put on the weapon, but, if honest, I do think there's some innate bias I have against polymer rifles too (I'm working through this though process through the Galil vs Galil ACE too). I guess it is what it is, but I am leery of polymer battle/modern sporting rifles for my needs. But the same doesn't apply to pistol carbines either. I don't know, but your mileage likely will vary...