Saheeli was pretty feast and famine for me. On one hand, she's a stronger Young Pyromancer since she triggers on more than just instants and sorceries. On the other hand, she can be a 3-cmc do nothing. She didn't fare well as the power creep dial went to 11 later in 2019 and through 2020 as more aggressive threats / better removal came along, not to mention more competition in Izzet.
Yeah I think if this card was mono-R or mono-U I'd play it, but taking a guild slot for a somewhat narrow card isn't ideal, especially when Saheeli the gifted is a more powerful and more flexible card.
My High Octane Unpowered Cube on CubeCobra
375 unpowered cube - https://cubecobra.com/cube/list/601ac624832cdf1039947588