I know I've shown Liu Xi transmuting quite complicated stuff before now (as you've had her do) but I'm generally happier with her controlling the primal elements. It feels more "elemental", and it has a mythic rather than scientific basis. Conversely, Alcheman, whose self-transformations are mystical, tends to root his abilities in actual chemistry.

Again it comes down to drama. A character who can simply turn an enemy into a pillar of salt (as Sersi could) is really hard to use in combat. Either you have to take them out right off to give the team a good fight or else find yet another reason why this enemy is shielded from the effect.

In Liu Xi's case I tend to assume that more complicated, actual chemical manipulations must take longer and be harder than "raw" elemental use. I don't know if Liu Xi can usually radically transmute (say earth to water) but if so I'd say that has to be pretty draining too.

Conversely, an ability of Liu Xi's that I really like is her sensitivity to the elements. It adds a nice unique perspective to investigations, enhances dramatic tension, and perhaps explains why the character can sometimes be a bit detatched from the world - she's effectively seeing tides and colours the rest of us can't possibly perceive.