So sunstreaker

Same thing but just in yellow

the right hip being weaker is definitely a thing
While as the white one the hip is literally flopping around sunstreaker's is at least holding its position.
But compared with the left side the right is clearly weaker.

As for the knee they are about the same. I think the body is just a bit heavy for the knees to hold.

But regardless of the weak hip or knees I would recommend a stand for them. The feet are a bit unwieldy due to the shape and now added articulation. The heel doesn't help too much as they don't extend backward where they are needed the most

>The head

No it is not spring loaded.
It is just pinned to the neck with an additional pull up joint for car to bot transformation.
The problem is there is enough room to tilt the head backward but the joints will force it back into its lower resting position