It is a fairly simplistic model, employing several simplifying assumptions, most notably the assumption that every fertilisation event may be classed as either self-fertilisation, or outcrossing with a completely random mate.
Thus the only model parameter to be estimated is the probability of self-fertilisation.
[1] The mixed mating model originated in the 1910s, with plant breeders who were seeking evidence of outcrossing contamination of self-pollinating crops, but a formal description of the model and its parameter estimation was not published until 1951.
For example, a weakness of the model lies in its assumption that inbreeding occurs only as a result of self-fertilisation; in reality, inbreeding may also occur through outcrossing between closely related individuals.
The effective selfing model relaxes this assumption by seeking also to estimate the degree of shared ancestry of outcrossing mates.