I mean, yes Obi-wan says he will become extremely powerful, but really he doesn't. The only thing he does is give Luke some advice that could be given to him through the X-wing headset anyway. So how is that powerful,
and where was he when Han died?
So the question is, why would forceghosts ever be seen as powerful and why do they disappear after some time?
From my understanding, it was never discussed whether Force Entities are powerful or not.
One becomes a Force Entity upon death, when he/she manages to deny the will of the Force. It is said that at some point Qui-Gon Jinn has traveled to a planet in which an extremely powerful race of Force Users lived on. These users were called Whills. There he learnt how to deny the will of the Force. After his death, Qui-Gon taught both Yoda and Obi-Wan the skill.
These Force Entities (AKA Force Ghosts) could not physically affect the world much the same as The Force (which could only affect the physical world via a Force Sensitive individual). Therefore - Force Ghosts are not powerful at all, since they simply cannot affect the physical world.
Having said that, please note that it is said that some Sith Lords have managed to learn similar techniques which in some cases allowed them to physically interact with their environment.
for your second question: I cannot remember any place in which it specifically explain why they remain visible only for a short period of time, therefore I am assuming that its their decision to disappear. Much like that it is their decision to appear in the first place.
Hopefully this answers your questions to your satisfaction.
