That was an ability and specific power, but id hazard a guess that doing those feats completely drained the user for days. And who says he cant? Flow walking was not a dark side technique yet Luke did not approve of using it, and later did not want his son to learn it or use it at first, yet conceded at the end. Again just because he has not used it, does not mean he did not have the ability to learn and master it.
As for palpatine, he was strong, heck way stronger then Luke at the time they clashed, but as seems to be the popular thing, his arrogance was his downfall.
Darth Nihilus, again yes he had the power to devour an entire population with the power of the force, but look at some of the other Jedi that were not as "powerful" seemingly, they all have their own strengths and weaknesses, corran horn was able to use the force in all ways but manipulation to physical items with telekinesis yet, Valin Horn had a natural ability to manipulate insects to do whatever he wanted them to do. Maybe not as impressive as devouring a planet, yet my point is is that nihilus might have only has that power be his most powerful force ability yet was a poorer casual force user, casual being push, pull and other powers. Heck using power in that access he might have depleted his entire "reservoir" of force energy and then some leaving him weak as it satisfied his hunger, leaving him vulnerable to death by force use as it strengthens his physical abilities. As far as i see it having played kotor, nihilus seemed to be at his maximum power, at least his power seemed to be most effective when he had the drop on his targets, but face to face encounters, (beings that had defenses in the force up and prepared for combat) seemed to be weaker and less effective.
maybe if he trained himself he could have been a "dark Luke" of his time in the measure of power and natural use of the force, but like most Sith, he believed his presence and power sufficient enough to never be formally challenged, or at least challenged and defeated until it actually happened.