Modern taxonomists have done a lot of lumping and splitting of 19th century (and older) taxa, mostly splitting of genera and lumping of species. We tend to lump the older species because 19th century (and older) mycologists often named a mushroom as new just because it was unusually large, or had an unusual color for that species, or had some other unusual developmental characteristic. There was another problem, too: many tropical and asian species were named by european mycologists ( Berkeley and Hennings come to mind) who had material brought to them by expeditions. Since they were unfamiliar with these fungi in their original habitat, and had never seen the range of variation to which they were subject, if a second (or third) expedition brought them the same fungus, they often failed to recognize it and gave it a second (or third) name. 20th century mycologists have put in a lot of hours cleaning up messes like that. That a lotta lumping
Modern mycologists have also split quite a few older "variable" species (like the honey mushroom), where people have long suspected that more than one species was being treated under a single name. In this case, modern mycologists are able to split the species because they have more sophisticated tools (primarily DNA analysis) available to differentiate the mushrooms. This sort of work gets more publicity, because people actually use the new names (and this annoys other people, generating still more publicity). By contrast, a lot of the old names that get synonymized haven't been used in a while, so no one cares; and in a lot of cases, names that probably should get synonymized aren't, because they aren't used anyway and no one wants to bother.
The changes that most annoy the average mushroom-hunter, however, are the segregation of new genera, based on (a) the ever-increasing number of species and the need to divide them into manageable groups and (b) ever-increasing sophistication in our methods of differentiating fungi. Maybe an example will help.
Linnaeus used one genus (Boletus) for all mushrooms with pores. He did this even though the genus Polyporus was available, created by Micheli. Think of it: all the Boletaceae and all the Polyporales in one genus. The mind boggles. Fries helped things out somewhat by resurrecting Micheli's genus Polyporus. He furthermore divided it into three subgenera: Favolus (with large pores that resemble a honeycomb), Microporus (small pores, more or less round), and Polysticta (no real tubes; just little round impressions in the fertile surface)
What followed is a miniature history of the creation/segregation of new genera in general. By 1850, there were six Friesian genera, mostly created on the principle of "one of these things is not like the other". There was Fomes, for all perennialpolypores (most of them hard and woody).
There was Trametes, for leathery things with fur on top. Favolus was preserved, but Hexagona was added, for things with even bigger hexagonal pores (most of them are just about the size of actual honeycombs). Polyporus and Polysticta were used as the garbage categories, for "everything else". Daedalea was already in existence (created by Persoon), but it seems to have taken quite a while for people to realize that Daedalea was somehow "like" a polypore.
I don't know why Fries didn't also include Nees' Boletus molluscus, shown here; but his other flagship Polysticta was clearly more of the same. In any case, several attempts were made at redefining Polysticta so that it could be distinguished consistently and productively from the other genera. Most of them developmental in nature (the entire cap grows before the pores start forming, versus the pores start growing immediately); Overholts & J. L. Lowe's (1953) Polyporaceae of the United States, Alaska, and Canada has an interesting summary of these attempts in its introduction. They all failed, however. By the early 1900s, Polysticta had been abandoned by all except a few fanatics like Lloyd, and its remaining species were all transferred to other genera.
I have synonymized several species on this website when the descriptions of each species available to me were identical in microscopic features, and differed in macroscopic features only in ways that are often due to environment. These synonymies are not intended to be statements about the absolute identities of these fungi, just about the realistic possibility of identifying them given the data available, especially when that data is as confused and contradictory as that on taxa such as Cortinarius argentatus and the members of Coarsely Fibrillose Inocybe.
Similarly, I have also included several species not in the books. Sometimes these are things that professional mycologists are working on and just haven't published yet; sometimes they are things that no one alive seems to know about yet. It's still fairly routine to find new species of mushrooms, even quite large ones. It's hard to say whether it adds more excitment or frustration to the study of fungi.