Definitions of simple interest differ, e.g. the naïve version here: What is 'Simple Interest'.
Simple Interest = P x I x N
P is the principal
I is the periodic interest rate
N is the number of periods
(an example is included below)
However, by contrast, insofar as "Simple interest is computed on the actual balance outstanding on the payment due date.", this is what is used by standard (actuarial) methods.
The simple interest (actuarial) method also differs from the Rule of 78s.
As described here: Rule of 78s - Precomputed Loan
Finance charge, carrying charges, interest costs, or whatever the cost
of the loan may be called, can be calculated with simple interest
equations, add-on interest, an agreed upon fee, or any disclosed
method. Once the finance charge has been identified, the Rule of 78s
is used to calculate the amount of the finance charge to be rebated
(forgiven) in the event that the loan is repaid early, prior to the
agreed upon number of payments.
So taking a simple example: a 12 month loan repaid early, after 9 months.
principal s = 995.40
no. months n = 12
int. rate r = 0.03 per month
By simple interest (actuarial) methods, using formula 1 (derived below).
repayments d = r (1 + 1/((1 + r)^n - 1)) s = 100
total int. t = d n - s = 204.60
However if the loan is repaid early, after 9 months, using formula 2.
x = 9
total int. t = ((1 + r)^x - 1) s + (d (1 - (1 + r)^x + r x))/r = 187.46
So the interest saved by repaying early is
204.60 - 187.46 = 17.14
If this was calculated by the Rule of 78s, with the finance charge taken as the total interest due for the 12 month loan.
precomputed interest f = 204.60
precomuputed loan = s + f = 955.40 + 204.60 = 1160
interest forgiven = f (3/78 + 2/78 + 1/78) = 15.74
So in this case it disadvantages the borrower to use the Rule of 78s.
Note the finance charge calculated by the naïve simple interest method in the aforementioned link: What is 'Simple Interest'
Simple Interest = P x I x N = 955.40 x 0.03 x 12 = 343.94
This is a long way from 204.60, but then the demo interest rate is quite high, accentuating the disparity. The naïve simple interest method is otherwise disregarded in this answer.
Demonstrating the interest calculations graphically, it can be observed that the interest payments calculated for months 10, 11 & 12 by the Rule of 78s are less than the simple interest/actuarial calculations.