Calling Ionescu slow is fine, but then comparing her to Taurasi is weird because she has been slow all her career and it hasn't stopped her from being called the GOAT by a lot of people. (Myself not among them, but she is unquestionably one of the greats).
Diana was an elite scorer, and that translates very nicely from college to WNBA level. SI seems like slightly more of an all-rounder, which traditionally doesn't translate quite as well, but she's smart, unselfish, and plays hard, so her ceiling is still potentially very high.
The fact that she's going to a bad team is very minor in the context of her overall career. It's unlikely they will be bad forever, especially now that Dolan is finally out of the picture.