The biggest single issue with polymer, plastic, rubber, and composite material in guns is ACCEPTANCE. Some of you old farts (like me) may remember when S&W introduced the Model 60 in 1965 ... the first production gun on the market made out of stainless steel. At the time, most gun owners were locked into blued guns with wood grips. We bad mouthed stainless guns with comments about wearing out faster, galling, looks, etc and just knew stainless guns would not stay on the market long. My how times have changed! 44 years later, stainless steel guns are by far the most popular handguns and are closing in on the rifle market too. It's getting where new blued guns are hard to find and in fact some models are only available in stainless ... think SP-101. Ruger has been the gun industry leader in cast stainless steel with their Pine Tree Casting factory.
Along came Gaston Glock in 1980 with his polymer frame Glock 17. By 1984 they were in full production and were introduced to the US market. In the early '90s, Glock pistols took over the market for law enforcement and are now the single biggest selling pistol world wide. Though 25 years isn't a huge amount of time, Glock proved polymer does indeed have a viable place in the gun market.
Here's some of the advantages with polymer frames: Polymer can be molded in just about any shape and can be machined to very exact tolerances. Polymer is much more resistant to body sweat, salt air, chemicals, and temperature extremes than any metal. Most things that will ruin a finish on a metal frame won't bother polymer at all ... like rust or corrosion. The color is molded in so if you get a scratch, it hardly shows, unlike a blued steel frame. Polymer is very strong and wear resistant .... more so than aluminum alloys and in some cases, better than steel. Weight is also a big advantage where polymer is way lighter than steel and rivals aluminum alloys. The cost to produce a polymer frame is a fraction that of steel or aluminum.
Many people don't understand polymer properties and fear excessive wear ... especially with the slide rails. First, polymer is "self lubricating" meaning you should not use oil or grease on slide rails ... just keep them clean and they will outlast aluminium and even steel. Again, most people don't understand how a pistol slide assembly interfaces with a frame. The barrel locks into the slide and because the sights are also on the slide, it operates as a "unit". If the slide fits loose on the frame rails ... no big deal and as the slide "wanders" a bit on the frame rails, the locked in barrel is still aligned perfectly with the sights. A loose slide-to-frame fit won't affect accuracy at all and might even improve reliability.
There are very few downsides to polymer. Some solvents will turn the surface white so you should avoid them. Carburetor cleaner, brake cleaner, and other non-gun products should be avoided in all guns but especially polymer frame guns. Some other products such as Loc-Tite, MEK, and super glue can actually "melt" polymers so they should also be avoided.
My biggest complaint about polymer grip frames is the grips themselves and the "one size fits all" concept. Because the grips are molded in the frame and are not removable, it could be a disadvantage for people with anything but average sized hands.
Being an old fart, I'm in the "blued steel with wood grips" era but I've learned to accept stainless steel guns and will probably learn to accept polymer too.