True vs physical/magical damage
A lot of people seem to think that true damage is the superior damage type as it ignores DEF and MR. However, it is harder to buff true damage. So while true damage has a "built in" buff, there are lot more buffs that the player can put in place for physical and magical damagers.
Most (but not all) true damage dealers have their damage scale equally from ATK and MAG. This makes the ATK UP and MAG UP buffs less effective than for physical and magical damagers. On the other hand, true damage will benefit from either one while physical and magical damage need their specific buff. These buffs increase ATK and MAG by 50 % respectively. So, looking at characters that don't have additional scaling from other stats (like DEF or HP), ATK UP will increase the damage for a physical damager by 50 % while it will only increase damage by 25 % for a true damager with equal ATK and MAG stats (and the same for MAG UP and magical damagers, of course). It should be noted that characters with additional damage from DEF or MR will need both ATK UP and DEF UP, or MAG UP and MR UP, to get a 50 % damage boost, just like true damagers will need both ATK UP and MAG UP for a 50 % damage boost.
Also note that there are some attacks that base their damage on such things as SPEC and HP (in many ways, such as own max HP, enemy's max HP and enemy's current HP) which are stats that can't buffed and thus those attacks can't be buffed as effectively as others.
Critical strikes seem to work the same way (double damage) for all damage types and is perhaps the best way to buff a true damager.
For physical and magical damagers, there is also PHYSICAL CHARGE and MAGICAL CHARGE, which greatly increase the damage output from physical and magical damage respectively. However, the buff is consumed upon first use. There is also an ability called charge which increases the damage from the next offensive ability. However, this is only available through two characters (Assassin Troll and Assassin Death Dog) and the Troll can only apply it to itself.
Another way to increase damage is to debuff the enemy. The most common way is with DEF DOWN or MR DOWN. Exactly how DEF and MR works is not known by the player community yet, but these debuffs do help. Even better are the buffs called ARMOR PENETRATION and MAGIC PENETRATION, which have a chance to be activated by equipping Piercing and Penetrating talismans respecitively and doing perfect hits as well as through some characters abilities (eg Samurai Eddies fury move). These buffs make the character ignore the enemy's DEF and MR respectively, making it akin to true damage.
There is also the Curse debuff that can be applied by the Pharaoh Dog and Pharaoh Dog King, which increases the damage taken from all damage types. In a similar fashion, the Ignite and Smite debuffs increase the damage taken from physical and magical damage respectively. Ignite and Smite are also more common than the Curse effect and some characters have a chance to apply them to multiple enemies as well. From what I've seen, damage is doubled against enemies who are Ignited or Smited.
On the other hand, it is also a little easier for enemies to protect themselves from physical and magical damage, as debuffing your ATK or MAG will affect those damage dealers more, and buffing their own DEF and MR will reduce physical and magical damage while it won't affect true damage.
In conclusion, true damage is more stable and reliable with less ways to vary both up and down, while physical and magical damage have more, as well as more common ways to be buffed, and stat buffs are usually more effective. Getting buffs in place may require some build-up but can give great payoffs which can be very useful against bosses.
