Both in cartoon and toys? In cartoon, his parts definitely "combine" to make him, where as other combiners like Arialbots and Stunticons, the vehicles attach to blocks that stand in for arms and legs. As a toy, its the same way: Devastator actually has feet out of 2 vehicles, whereas the other combiners have "stands" as their feet with the vehicles on top. So why was his design so different from the others?
One explanation I can think of for both the fiction and the toys is that Devastator was different from the rest because he was the first combiner ever - a prototype for subsequent combiners, if you will. The Decepticons (and Hasbro) only formulated the idea of Scramble City-style combiners later, after evaluating the finished Devastator and improving on the mechanics of combining.
Animationwise, It might have just been easier to animate. They designed Devistator & Sunbow heard complaints from animators how it took longer to make the legs look like specific things. So when they designed the others, they made a regular robot and just drew the car/plane parts onto the legs/arms.
In the recent Collector's Club comics, Bruticus was actually first (prior to his components' brains being locked in the detention center). Other 'Cons used the tech to make Devastator- however, they built him too hastily, and thus he is imperfect/different compared to many other combiners.
The toy is different because like all of the '84 and most '85 TF', they were originally from the Diaclone line of toys.
dino king and raiden and lio kaiser make out of 6 members tollay 5 or 6 members combie in into giants warriors
The original Diaclone mold was made before they established that a combiner usually 5 components, and not six.