Alginate has been used for many years to take impressions for study models, opposing models, templates for provisionals, and other non-precision purposes. It is easy to use, it is relatively inexpensive, and its viscosity can be modified. But it is also somewhat messy and, in most instances, must be poured shortly after the impression is made.
VPS materials have been created specifically as alginate substitutes. They typically are less rigid and usually cost less per milliliter than conventional VPS products. However, their viscosity cannot be modified just in case you need a thicker material to register, for example, a high palatal vault. In this cases, you could use a heavy body conventional VPS, but this could be very expensive.
And, even though VPS materials are very capable of registering minute detail, their use as an alginate substitute can produce wrinkles and voids, especially when you try to capture a full arch under less-than-ideal conditions such as the teeth being too wet with saliva. While these materials are reasonably hydrophilic, they don't come close in this property to alginate. Furthermore, these materials tend to lock into undercuts and open gingival embrasures more tenaciously than alginate.
In addition, a large maxillary tray could require virtually all the material in a typical dual-barrel cartridge, which can also make these impressions quite expensive. This is why using the bulk cartridges in a mixing machine makes much more sense with these materials, since the mixing machine versions are much less expensive and make it infinitely easier to fill a full arch tray. Ask any dental assistant and he/she will tell you that pumping an impression gun to fill a full arch tray is not fun!
Ever since the first "hydrophilic" VPS was introduced a number of years ago, there has been a race among the manufacturers to create it with as much hydrophilicity as found in a PE. The thought was that the more hydrophilic a material was, the less likelihood that fluid in the sulcus would distort the impression. The hydrophilic material will merely absorb the fluid and continue with its mission. Note that hydrophilic properties in VPS products need to be additives, since these materials are not inherently hydrophilic as they are with PE.
Nevertheless, since these materials do not have to be poured for two weeks and can usually be repoured at least several times, they are definitely increasing in popularity. Just be sure to recognize that since they are not inherently hydrophilic, stone can bead up like water on a freshly waxed car. Therefore, using a surfactant and small brush goes a long way to prevent voids in stone models.