Untargetability is mainly useful against single-target attacks.
Examples: Caitlyn auto attack, teemo’s Q
AOE spells like Caitlyn’s Q or TF’s Q can still hit you while you are a wraith.
And Seena’s E is a camouflage. Senna and allies in her E will be revealed by a revealing action: enemy champion moving close enough to Senna, a control ward, Quinn W, or abilities/effects that grant true sight like the Fountain and ignite (idk about ignite, haven’t heard any mention of the interaction and I haven’t seen any gameplay where a Senna or an ally was revealed by an Ignite)
Don’t confuse camouflage with invisibility.
’s R and Vayne’s R are good examples, their invisibility can’t be broken by simply walking near them, only the fountain and a small amount of other abilites/summoner spells can reveal them.
The entire reason why the Boards got so worked up over Senna (before she was available on the PBE but after her abilities were announced) was because they confused camouflage, invisibility and untargetability lol.