In a setting where a thought or an incantation can set one's weapon on fire, launch a kinetic projectile, or transmorph one into a nightmarish stagman, there's nothing brittle to be questioned.
This depends on your take on the limits of the possible, which others were discussing earlier with reference to the question of God's omnipotence (is He only able to achieve the sum total of all that is theoretically possible or does that include the impossible as well, thus exceeding the sum total, as it were?). A fantasy setting is bound by a set of more or less elastic rules that ensure a minimal degree of self-consistency, and PoE is no exception. As long as the setting subscribes to these self-promulgated rules (to the best of its ability, and not without transgressing them every now and then), not everything is consistently possible
within its confines. 'Absolutely anything can happen, at the drop of a hat' is closer to surrealism than to fantasy, which abides by a far stricter set of laws, no matter how fanciful they might seem from the vantage point of what we call 'reality'. So it is perfectly coherent for the characters that dwell in this fantasy universe to become disillusioned with their beliefs (Edér relative to his brother, Durance relative to Magran, Sagani relative to Persoq, etc.) so long as these beliefs seem(ed) consistently real
to them.
Now if your argument is that this intradiegetic disillusion rings existentially hollow for the player – for us living, breathing, thinking human beings – when set against that of e.g. Kris Kelvin, you are absolutely correct. It would be risible to pretend that PoE conveys so much as a fraction of the depth
Solaris does. But my initial contention was different: it was that the disenchantment of the world is an intriguingly self-defeating theme for a fantasy CRPG, and that PoE therefore achieves something different from its peers, which is notable in its own right, regardless of how one feels about the end result. What's more (and this is mostly speculation on my part) such disenchantment is doubly striking in the context of a game that explicitly harks back to a (then-)bygone subgenre of CRPGs, one that seemed far more confident in its ability to marvel and amaze us, to have us believe in the unbelievable, be it only for a spell. There's something deflated about PoE's mood, as though it didn't buy into its own tricks, which partly explains why so many Codexers loathe it. Yet it just so happens that this tension is quite interesting to me, drawing me back. And while it's possible to experience similar feelings of stunted nostalgia while playing other Kickstarter-era CRPGs, PoE cops to its dispiritedness and turns it into its a vibe of its own (regardless of whether this was a conscious decision on Obsidian's part or not, and I suspect it wasn't), which I quite like.
Could it have been done better? Unquestionably – much better, even. But that it was done at all when set against the body of CRPGs that are currently available to us is no mean feat. In conclusion, everything is shit.