Hehe, martial art question in worldbuilding! Let's take historical short staffs and long staffs separately before treating the double knob staff.
The longest staff/staff-like weapons that have been used historically are the pike poles, halberds, the Chinese spear poles and the Japanese naginata. These were mostly not targeted at people but at horses during warfare. These are invariably weighted at one end and are not expected to be balanced at all. The knife or spear point is heavy, bearing these at one of the two ends of a 7-10 feet long pole: the metal blade (for slicing and 'poking' power), the tying rope and the extra tassels etc (to cause maximum internal damage as well as slip out of flesh quickly while ripping it out, brr, boy, am I super glad we don't live in those ages!). This is intentional since the unbalanced end makes it easier to wield - your strength and gravity work together to keep the long staff in constant motion and give it its thrusting power. These poles are usually flexible to prevent breakage. Check out the special martial arts practice routines devised around these weapons to see what I mean. YouTube has several (apols, can't seem to link anything in the post on a mobile device)
There exist double balanced blades on both ends of a long pole but these are not as popular as single ended ones. I suspect the double ended ones would look flashier since it would be easy to do rotational motions and slices with them. But these would need greater strength to thrust and change direction while wielding. (added later:) Also, with a 7.5-foot chinese spear pole, about 3/4ths of the moves hold the pole not at the balance point halfway, but with one hand a bit above one end and the second 1/3rd of the length up, the intention being that there should be 'a good length to poke and slice with'. 358
Any sharp object on the shorter end would be a major irritation and require extra care not to cut yourself with. It can be done, like with a double naginata, just more trouble than it's worth. Since this is a knob, not a knife, doesn't have a cutting problem but might have a manoeuvering problem if the weight is too heavy.
Short staffs usually do not have any spear points and knives at the end. At short range, the points are difficult to wield and attack with while protecting yourself at the same time. So usually the staffs at 4-6 feet length are unweighted at the ends except to take advantage of 'gravity assist', you could have a knob at one end acting as a bludgeon. Staffs at this length are more of blocking and bludgeoning weapons than slicing or thrusting weapons. Also they can be 'fatter' or more inflexible than long poles for the weight of it as a weapon that can break bones and hurt. As an added advantage you can use them for walking sticks, to chase away dogs and hang up washing (haha, but this is exactly what ancient travelers and wandering monks did, staffs are delightfully multipurpose).
So double knobbed staff okay, bludgeons well and rotates well due to the balanced nature. No gravity assist but sheer human strength can compensate for it at 4-5 feet length, as a thrusting weapon as well. No obvious disadvantages. Just keep the knobs not too big or heavy.
Curiously, a double knobbed staff looks a bit like a long bone...
(added after some more thought): The joint where the knob meets the staff pole would be prone to breakage on impact if the knobs are exactly spherical meeting a thin pole. Perhaps the knobs could meld in like the ends of a bone. 523