An arranger has at best 2 layers/splits for the left hand and usually only one (If any) for adding bass pedals, organs have multiple layers/split points all independent of each other, plus a number of organ manufactures allow you add even more.
An arranger requires you to play the chords in a way that it understands, with an organ you can play any way you want, which is way more flexible.
You can add manuals and pedals to an arranger (The TRX system for example) however it requires some serious work arounds, and still falls well short of the organ flexibility.
An arranger has less real estate to add real time controls compared to an organ.
There are more differences which is why they are 2 separate instruments. (Even though the arranger is an offshoot of the organ)
Anybody that says there is one instrument that can do everything, I am afraid does not live in the real world.

Bill