The meaning of most statements in English is dependent on context. Any time you have a statement that "X is Y", and "X" does not identify a specific entity, then identity of X must be inferred from context.
Consider first a statement like "Rainforests are in danger." This simply says that some subset of all rainforests are in danger. From tone and meager bits of context one might infer that the meaning is "all rainforests are in danger", but that meaning is not explicitly stated.
When one says "The rainforests are in danger," the use of "the" tells the listener that context must be consulted to determine which specific rainforests are being indicated. If the surrounding discussion is about, say, South America then it can be inferred that "the rainforests of South America" are being indicated. However, if there is no such context then the "wider context" ("global", in fact) is the general knowledge that there are rainforests all over the world and hence the term is interpreted as "the rainforests of the world".
With guitars, if someone says "the guitars sound lousy" there is no standard "wider context" such as there is for "rainforest", so, outside of, say, a discussion about the local rock band, there is no reference point for "the guitars", and the statement sounds, well, silly. If one says "the guitars are musical instruments" it doesn't sound quite as silly, but it's still clearly not right, for the same reason.
In general, when "the" is used with a plural noun to indicate a generic class, there must be some available context so that the reader/listener can resolve which entities are being identified. Sometimes, for some classes, this "context" can be "global" in nature and not be the immediate context of the sentence, or even the wider context of the article or the technical domain of the article. In other cases there is no such "global context", and a generic "the" will end up being ambiguous.