First of all, I don't see this as a problem.
Eventually, you will find all the spells. Saying that IE does not provide you with enough spells to push through is untrue.
Second, we're talking about shit on systemic level. And on systemic level, IE Priest has all spells, while Deadfire priest can have 1 spell and that's it.
You also ignored your other statement: "In IE every class is a well rounded package doing everything you need"
Clearly, that's just bull.
Not if you get out of your mind the idea of some classes being op than others.
In IE games every class is designed as a package of abilities and tools which will all eventually be available to you. Eventually, thief is going to get the full power of backstab and master most of the required skills, wizard would have all spells, and fighters have both offense and defence.
The biggest difference between one player and another would be mostly how he uses classes. My pure druid would solo whole trilogy using my knowledge of the spells. Anyone else's probably wouldn't. Yet the difference in "build" won't be huge at all.
How exactly is this a trap design, compared to Deadfire's design where you can pick an extremely overpowered thing (Whisper of Treason) or some shitty one?
The amount of choices increases the amount of mistakes you can make. In IE "good build" for a fighter is not dumping STR, finding some good weapons & armor that's it. In PoE it is about stats, armor you wear, abilities you pick, using or not using dual wielding (hint: use dual wielding), etc.
It's more fun to fine tune your character like this, but the more variables you include, the bigger would be the gap between this
and other characters.
So the fuck people continue to talk about "traps" in IE games IDK. It seems to me like the problem was always exaggerated to begin with. They are nearly not as "trapsy" as many other RPGs I know; in many ways they are actually very good as your first baby steps RPGs - I know, because I beaten them young and without any knowledge of what the fuck THAC0 is while being shit at english.