A lot of the Gen 1 oddities are because of the extremely limited move pool.
Gyrados as Water/Dragon for example, would have had no weaknesses as there was only one Dragon move, Dragon Rage, and it had a set damage value that ignores weakness.
A bit less so with Charizard, because Rock Slide existed, and Golem and Rhydon were decent Pokemon, but a Fire/Flying starter would be much better than the other two.
However, one Pokemon from Gen 1 always bothered me. Blastoise should have been changed to Water/Steel when Gen 2 came out. It bothers me that Venusaur and Charizard are dual types, but Blastoise is not.
Everything was destroyed by Psychics in Gen 1. Specifically Alakazam, who is hilariously overpowered. And that's not even mentioning Mewtwo. Gen 1 Mewtwo was probably the strongest Pokemon the series has ever had until Gamefreak got high on some wack stuff and invented Mega Rayquaza.