X may cause … X is limited by … X suffers from … X is too expensive to be used for … X has accentuated the problem of … the performance of X is limited by … X could be a contributing factor to … the synthesis of X remains a major challenge. X can be extremely harmful to human beings. research has consistently shown that X lacks … the determination of X is technically challenging. a major problem with this kind of application is … current methods of X have proven to be unreliable. these rapid changes are having a serious effect on … X can be adversely affected under certain conditions. accounting for these varying experiences is problematic . observations have indicated a serious decline in the population of … | 
