the title actually says it all. if the expansion adventure cards have 30 cards all with the same symbol it makes them very hard to use in custom scenarios. if they would be split in 2 or 3 parts with different symbols it won't hurt the expansion in any way, but it would also make them great for designing custom scenarios. it would even help ffg yourself if you want to re-use them in future expansions.
the split can, but does not need to be thematically. just having 2 identical sets with slightly different symbols would be sufficient, so i could use only half of them in a custom scenario if i wanted to.