Ascendant = other class?

What the difference between playing as any class and doing the same thing as ascendant,
I mean srs necro = ascendant srs is like 99% same?

Do you mean in terms of an ascendancy class?
So if i like go try to play poison srs scion its will be
(a lot or not much) inferior to an same necro p srs,
Like i will not get that minion buff also buff you thing?
In terms of advantages and disadvantages the scion is more like same class but -some amount of power?

Depends on the build, gotta work around what the Scion gives you.

Can be better due to Scion having access to different nodes, can be worse due to Scion being (overall) less specialized than main classes.

Heres an example build from a Ranger moved to Scion, in this case seems to a bit better, but only way to judge is by trying it yourself.
https://www.pathofexile.com/forum/view-thread/3343136