Consider the bits of evidence (I'm going to call the Rotten/Fungal Forest non-Environment Forest Wastes from now on):
Background
Different environments usually have different background colours, but not always. Wastes, Hell and Forest Wastes all have the same background colour, but so does Haunted Wastes, which is definitely a distinct environment. So there's not much to deduce from this. You could point to the trees that appear in both Wastes and Forest Wastes, but this would really be clutching at straws.
Music
Different environments have different music. Wastes, Hell and Forest Wastes all have the same music (but Haunted Wastes doesn't), which is an argument for classing them all as one environment.
Functional scenery
Now we're talking about things which actually affect gameplay. Two of the three campaign Wastes levels feature graveyards, the other featuring a magma field instead. Neither of these feature in Hell or Forest Wastes, but they each have their own functional scenery. Hell features thermal fissures whereas Forest Wastes features poison pools. Of the two, poison pools have the greater effect on gameplay, but both are equally distinct. Overall, there's not much to deduce from this.
Enemies
This is the big one, and the main reason why I think that Forest Wastes is more deserving than Hell of being classed as a distinct environment.
Different environments can feature some of the same enemies. For example, even just within KR, bandits, brigands, gargoyles and the basic spiders appear in multiple environments. Nevertheless, new environments universally introduce new enemies: the various trolls, yetis and winter wolves in the Mountains; the various demons and necromancers in Wastes; zombies, rats, fallen/spectral knights, werewolves, abominations and hags in Haunted Wastes.... It's these stock enemies that define the character of the environment.
Now, consider how Hell fits in here. The new enemies are demon legions, flareons and gulaemons (you could also make an argument for counting cerberus). That's not very many new enemies compared with other environments, but more importantly, none of these are the stock enemies that are common in the early waves. The first five waves of Pit of Fire see a total of two legions, but aside from that consists solely of demon spawn, demon lords, demon hounds and demon imps which are stock enemies in Wastes. Pandaemonium is more of the same - this time there are three legions in the first five waves but aside from that it's Wastes enemies only.
Constrast this with Forest Wastes. They introduce more new enemies: husks, noxious creepers, mutated hatchlings, tainted treants, swamp things and rotshrooms. More importantly, though, these are mostly stock enemies which dominate the levels. Rotten Forest begins with husks as new enemies, but admittedly there are quite a few spiders, demons and gargoyles thrown in on early waves. Fungal Forest, however, does not feature a single enemy that appears outside of the two Forest Wastes stages. From a gameplay perspective, that makes Forest Wastes more distinct from Wastes than, say, Wastes is from Mountains.
Conclusion
Personally, I would class Forest Wastes as a separate environment, and class Hell with Wastes, but you can certainly argue that they are all one environment. I find it hard to see an argument for Hell, but not Forest Wastes, being a distinct environment.
And yes, I appreciate that none of this matters
