Actually there are no disadvantages to periodic table ( the modern periodic table)
As prakriti said that hydrogen is kept in alkali group even when it shows properties of both alkali metals and halogens, hydrogen is not kept in alkali group. Hydrogen has been given a special place at the top of the periodic table. It is kept different from both alkali metals and halogens.
Now I m taking the case of actinides and lanthanides.
They both are kept seperate from other elements below the table only for the sake of conveinience. They are kept aside so that periodic table is uniform.
Hope my answer satisfies u