Devastator is an immensely powerful warrior, and in that sense the combination of the Constructicons into a merged being is a huge success. Mentally, on the other hand, he's very much less than the sum of his parts. Devastator's primitive psyche is limited to only the thoughts and actions on which all his components can agree, which results in raging bouts of destructive fury. Given the sheer power Devastator brings to these rages, Megatron has no complaints.