How Damage Over Time, Damage Each Action and Nukes work. With Numbers!
Let's say we have an ability ABILITY which deals XXX damage.
We can clasify that in 3 types of damage;
- A: ABILITY deals XXX over Y seconds. Z sec Cooldown
- B: ABILITY deals XXX/E damage each time T happens. Z sec Cooldown
- C: ABILITY deals XXX damage. Z sec Cooldown.
We consider XXX, E and Z as positive intergers, and T a boolean subject (Each time TRUE happens it deals XXX/E damage). Min(W) = 0.6*Z , Dam(W) = Damage dealt by W each Z sec and MaxDam(W) = Damage dealt by W each Min(Z)
As we see, there are 3 different cases. Damage over time (A), Damage when a certain situation (B), Damage (C).
The easiest case, C, is very linear. It deals XXX each Z seconds. So Dam(C) = XXX As in this game there is ways to reduce Z, Max(Dam(C)) = XXX
The next case, A, is something more complicated. We have to consider 3 cases. [Y is lower than Z and Min(Z), Y is lower than Z and higher than Min(Z), Y is higher than Z and Min(Z)]. In the first case, Dam(A1) = XXX and MaxDam(A1)= XXX. In the second case, Dam(A2) = XXX and MaxDam(A2) = (XXXMin(Z)/Y). In the third case, Dam(A3) = (XXXZ/Y) and and MaxDam(A3) = (XXX*Min(Z)/Y).
The lasst case, B, is the most difficult to calculate, as Dam(B) = XXX*(T/E) and MaxDam(B) = XXX*(T/E). This damage is harder to balance as it depends in how easy is T to happen.
To sum up, C cases will always deal more or equal damage than B cases, and B has the opportunity to deal more damage (T > E), equal (T = E) and lower (0 <= T < E) damage.
So, champions which dont benefit all they could from CDR because have B2 cases in their kit (B3 or B|C or C|B cases don't qualify as they don't lose damage over Z->Min(Z) by building CDR or are dependant on a T); 
