Dobereiner's system of triads was not useful.

Dobereiner called some groups of three elements which show similar properties as triad. 

In these triads atomic mass of middle element was approximately the mean of the atomic masses of other two elements. 

Dobereiner could identify only some triads from the elements known. Other triads did not obey Dobereiner's rule. 

Hence, the system of triads was not useful.