Honestly, I wouldn't know.....I'm familiar with a couple, but I can't say anything about 'results!"
By the same token, though, one could ask why there's more than one yidam, right? Yidam practice aims for the same result, ultimately, though methods differ, and perhaps even mundane siddhis may differ....
Theoretically, all LaDrups should result in the state of Vajradhara--that's the "Result," the Absolute Result, should one practice perfectly and purely. Guru Yoga, the path of Devotion, is also the means for attaining the "blessings" of the lineage, or, if you wish, of particular gurus in that lineage.
From the POV of relative truth, Gampopa was a different person than Marpa, who was a different person than Milarepa, etc. On the path of practice, the methods differ. Milarepa's Guru Yoga, for instance, relies on Dorje Phagmo as yidam, and on Mila as "In-Front Generation," and there are supplications, as well as instructions regarding Nature of Mind, which are included. Marpa's practice has outer, inner, and secret levels, and Hevajra is involved.
Gampopa was a (Kadam) monk. Marpa was a Gentleman Farmer. Milerapa was a naughty guy, or so the histories say, who became the greatest yogi in Tibetan history. Different people have different propensities, and may be drawn toward different types of teachers.
"Absolute Truth is not an object of analytical discourse or great discriminating wisdom,
It is realized through the blessing grace of the Guru and fortunate Karmic potential.
Like this, mistaken ideas of discriminating wisdom are clarified."
- (Kyabje Bokar Rinpoche, from his summary of "The Ocean of Definitive Meaning")