Dragons are versatile units that hve several functions.
They have the widest melee range of the 3 melee fights, having a larger melee reach then Berserker and Knight classes. They also have a ranged attack which is somewhere between the Ranger and Ice Mage in terms of range. They switch ranges to suit their needs. Their ranged attacks come with the added bonus of burning the enemy. With care and precise placement are able to fully cover a lane by themselves late game and do not always need the support of unit such as the Healer to cover them. They are also capable of taking down bosses like the Knight.
However, they are the slowest attacking unit of the game. Their lack of speed is made up somewhat by their "Roar" skill, they can cause a enemy to freeze in position allowing time for another attack to regenerate and strike.
The biggest issue with them is how costly they are to summon and boast. Often to place 1 and boast it to maximum costs more then two units from the cheapest classes, making placing them a decision not to be taken lightly. Their fire also cancels out the Ice Mage freeze abilities, so they are not suitable to be placed side by side.
They also cannot use weapons and do not wear armour. Their lack of armour can overcome by their point boasts to "Fatten up" and lack of a weapon by their "Strength" skill.
When leveling up their skills, one may notice that they have one of the sharpest increases to states for each skills, however, their skills do not have many extra attributes that can be added. In fact "Bite" is the only skill that has additional upgrades. Their "Nom" skill means they do not have to deal with weaker enemies at all and Bite can have poison added to it, meaning even if the foe walks away, they make continue to take damage. However, the overall lack of upgrades can result in limited tactics at times.