It really depends on your ear structure. Of course CM would be the best possible scenario but there's also the risk of a refit and unbalanced sound. I owned a pair of UE 18 Pro and I needed to refit twice to get the sound right. I honestly feel universals like SE535 and W4 fit well to my ear that isolation is not even an issue. I believe the majority of people should have "normal" ears and that universal should fit fine.
I don't dis custom as I would love to get a pair someday. However, I fall into slick530 category that universal fits fine for me with the Comply Tips, so I don't really need to go with the trouble of custom IEMs.