For instance, Financial is an ideal trait in this regard - it has one effect, nobody has ever complained that it needs more, and it's more or less universally loved by players.
True, but it's a very telling example: Players love the trait that provides extra commerce, which can be translated into extra research, extra culture, extra espionage, maintaining a bigger army, maintaining more units abroad, upgrading units, paying for higher city maintenance, rushing buildings in cities, and can be traded away to other civs in exchange for resources. The trait effect may be singular and narrow, but the trait output is incredibly broad, and happens naturally enough that the civ doesn't have to specialize around it.
In contrast, Philosophical's output is very narrow: one of 6 units. They can be applied for a variety of effects, but you have little control over which of those 6 units you get, since that's random, the bonus is sporadic and doesn't always arrive on time (eg storing away a great merchant until you can build a glasswork, or getting a great scientist/artist after all the era's great works of science/art have been claimed), and requires you to invest into building wonders/using specialists, even if doing so isn't otherwise optimal, since 50% of 0 is still 0.
As for "hit or miss", that's what traits are supposed to do, they are supposed to inform your gameplay style to a certain extent, not accommodate whichever play style you have.
I agree with that, but I think they still need to be flexible enough to work across multiple types of empires/starting locations. Financial is great because whether you're near to coasts, jungles, plains, or forests, there's typically some improvement or resource that lets you access its benefits. But if you're a philosophical leader but lack a good landscape to use lots of specialists (eg low food outputs), or are under constant threat forcing you to build units instead of running scientists, you have less yield from the trait.
Bilquis starting on a coast with limestone in her starting cross is going to do great! Bilquis starting inland without much production to get some initial great person generation is going to do poorly. And what's that? Her neighbor is Alexander the Great, who has Copper in his starting cross? Let's hope Bilquis has good insurance...
I don't expect traits to allow any civ to triump in any circumstance, but I think civs should be able to utilize some aspect of their traits to strong effect in most situations.
The core issue with "+50% GP rate" is that it doesn't translate into "+50% more GPs", and it never did due to the way GPs work, but psychologically it is registered as if that was the case.
Your line of thought here does raise an interesting question: Has anyone (including Fireaxis's team, hah) run benchmarks to see how many additional great people Philosophical gets on average?
My original point was more that availability of specialists/wonders informs a lot of Philsophical's yield, and since great merchants and great prophets are the more common early great persons*, the trait disproprortionally yields more of them than great scientists/engineers/spies, making it an even narrower effect than intended.
But to test my own theory, here are some numbers. Great person generation by type and era, excluding religion-based wonders and wonde-based specialists:
Great Prophet - 4 wonders, 1 specialist (temple)
Great Merchant - 2 wonders
Great Artist - 1 wonder
Great Scientists - 1 specialist (Library)
Great Engineer - 1 wonder
Great Spy - 1 wonder
Classical numbers:
Great Prophet - 1 wonder
Great Merchant - 2 wonders, 1 specialist (Market)
Great Artist - 4 wonders, 1 specialist (Theater)
Great Scientists - 3 wonders
Great Engineer - 3 wonders, 1 specialist (Siege Workshop)
Great Spy - 1 wonder, 1 specialist (Courthouse)
So in these two eras a civ gets a guaranteed specailist of each kind, which is more balanced than I thought (though I think practical gameplay encourages using some over others). Philosophical leaders trying to maximize great person points are likely to get more ancient era wonders, which leads to more great prophet and great merchants. Classical era unlocks a lot of great scientist wonders, moreso than I realized, but my anecdotal experience is that AI civs love to rush those wonders, so as a player they rarely contribute to great person generation. This might be an artifact of difficulty settings, though, and in that regard an intended effect.
So my case is weaker than originally thought, and I'd argue the practicaly of Philosophical in its current state varies by difficulty level, which may or may not befit your wants for difficulty level impacts. I could se arguments for and against that.
So that it'll turn an into even more of a kitchen sink trait!
Honestly, as a trait it could use a rexamination. While I like the trait for the +2 total happiness from barracks and arsenal, that particular bonus always felt a bit hacky, as if there wasn't any better ideas so extra happiness was added in. It would probably make more sense for Imperial to reduce city maintenance or something as a primary effect, and for the great general rate to belong to militaristic/conquerer. I guess GG points made more sense for Imperial before conquerer came along.