draw back of DGM

Where on earth did you read that?

It does certainly ignore the effect of personal tax (and capital gains tax) on the investor. Higher rate tax payers would end up receiving less (net of tax) than the dividend actually paid. This would only really be of relevance if a substantial number of the investors were paying at higher rates.

(Even then, it is not a drawback of the model at all – it simply creates a problem as to what numbers we use in the model)

However, we always ignore personal taxes in Paper F9!

It is wrong and is a silly thing for them to have written (although apart from those six words the rest of their answer is correct).