I agree with that, because once you know the rule there are actually almost no exception to it. The trick to know what's the radical of the verb in the Present Subjunctive is looking at the radical of the 1st person plural in the Present Indicative, and then applying the terminations according to the conjugation. If the verb is regular, the radical in Subjunctive is just the same as in the Indicative. But even for almost all irregular verbs this rule works. For instance, avere in the 1st person plural Present is abbiamo. The radical then is abbi-, and the conjugation is abbia, abbia, abbia, abbiamo, abbiate, abbiano (2nd conjugation). Other examples would be essere, stare and fare.
Some few verbs, however, take the radical from the 1st person singular for all persons except 1st and 2nd plural, in which they take the radical of the plural from the 1st person plural. In the case of morire, it's muoio/moriamo. Then the radicals are muoi-/mori- and the conjugations are muoia, muoia, muoia, moriamo, moriate, muoiano (3rd conjugation). This is also the case of capire, tenere and dovere, for instance.
I find http://www.italian-verbs.com/italian-verbs/conjugation.php?verbo=morire invaluable for conjugating verbs.
Have you seen this? https://forum.duolingo.com/comment/8783716 and you could look at the subjunctive lessons in this list: https://onlineitalianclub.com/index-of-free-italian-exercises-and-grammar-lessons/