
A thematic weakness but not much of a threat. Looking at the two elements, the effect and the discard condition, we get:
The effect: Luke is removed from his location and moved to the reverse side of Dream-Gate, where he can sit in isolation and think about what he's done. This can curtail 0-3 of Luke's actions, depending when you draw it, since he can still do things like draw cards, take resources, and play assets or events that target himself. It even disengages Luke from his enemies! Once in a very great while, this might stop him from supporting a fellow investigator at a crucial moment, but most times this is very mild.
The discard condition: Either investigate a 6 shroud location to get to the good Dream-Gate or wait until the end of the turn and take 2 horror. In either case, Luke then moves to any revealed location. Since Luke has 9 Sanity, this is almost like a slightly-damaging extra use of the Gate Box, and, in play, this has almost always been a welcome draw.
All in all, this is a way below average signature weakness. The only way it could be less of a weakness is if it just said "add a charge to the Gate Box."