When a certain company first brought out their artificial range I must admit to being a bit concerned.
Plastic is widely regarded as a pollutant the world over, particularly lost and discarded stuff that floats off ending up in our oceans. Often it tends to breakup into ever smaller fragments which are then ingested by fish, birds and sea creatures.
In the lake they simply persist 'whole' until the millennia of microbes and sunlight breaks them down - which isn't necessarily a bad thing, but it depends on what fillers are used. Polymers are not regarded as toxic per se, but there may be toxic residual chemicals, chemical additives and degradation products and fillers that can leach out as they are not bound to the plastic polymer. There are literally millions and millions of different types of plastics and composites produced for use in a whole range of consumer and industrial goods and situations throughout the world. Silicon is for the most part inert, it will pass through if ingested but it is much more likely to be felt by a chomp down as inedible and immediately rejected by fish. The smaller stuff that has broken down over time (or by the constant battering of waves) can be a bit more concerning when it gets into the food chain. Most 'artificial baits', bait stops and other small items should (but not always) be produced from similar safe and inert materials like silicone, but sadly most of what we use are imported unregulated without restriction or accompanying MSDS documentation.
One of the main chemicals used to produce plastics is bisphenol A, or BPA, an endocrine disruptor that is prevalent in a vast number of widely used products, not least of which are plastic food and beverage bottles and the lining of metal cans. Microbes, heat and sunlight, repeated washing or immersion, acidity, and alkalinity causes the BPA in plastics to leach out. BPA leaches into our groundwater from all the plastic sitting in landfills too. But by comparison of what is lost in a river or lake by your average angler, I would say its pretty much insignificant in the grand scheme of things.
Bioplastics - its the future!