I will enumerate all of Mr. G.K. Chesterton's (the man who wrote the book on Orthodoxy) differences here:
It is a "purely spiritual" form of Christianity.
the essential difference between Mrs. Eddy's creed and mine is that she anchors in the air, while I put an anchor where the groping race of men have generally put it, in the ground.
FAITH HEALING AND MEDICINE By G. K. Chesterton From the column Our Note Book, The Illustrated London News. October 22, 1910
And, like the gnostics of the first and second century:
In short, the first and last blunder of Christian Science is that it is a religion claiming to be purely spiritual. Now, being purely spiritual is opposed to the very essence of religion. All religions, high and low, true and false, have always had one enemy, which is the purely spiritual.
And the Manichaens who came after - and Augustine firmly opposed (after converting to Catholicism from the faith):
Christian Science may or may not start with the assumption that God is in his heaven and all is right with the world; it is a subject for a respectful debate with Christian Scientists. But Christianity emphatically began with the assumptions that God is in his heaven and all is wrong with the world; and from those two things the whole Christian theory proceeds.
BROWNING AND THE AMERICAN OPTIMISTS - By G. K. Chesterton From the column Our Note Book The Illustrated London News November 22, 1930
It denies death (all death including the part of the Creed where we say "Jesus died and was buried"
Should we passionately asseverate that something was as true as death, even if we all joined the religion of some Mrs. Eddy, which declared that death is a delusion, imposed by a conspiracy of undertakers?
THE DESTRUCTION OF LIBERTY - By G. K. Chesterton From the column Our Note Book The Illustrated London News October 06, 1934
And, while not a part of the faith, it denies human reason in the sciences, which is what Pope John Paul II would call fideism the twin heresy of rationalism.
The very system which would deny and destroy all physiological science, all medical science, all anatomical and surgical science, still calls itself Christian Science.
SCIENCE AND THE DRIFT TO SUPERSTITION - By G. K. Chesterton From the column Our Note Book The Illustrated London News November 13, 1920
As well as the twin heresy of materialism.
For the Hygienist is urging that if you take care of the body the soul will benefit by it; while the Christian Scientist is urging that if you take care of the soul the body will take care of itself
THE CRANKS OF THE HIGHER THOUGHT - By G. K. Chesterton From the column Our Note Book The Illustrated London News August 15, 1914
And, frankly, it is wrong to categorically deprive sick people of help
I think it both crazy and cruel for a follower of Mrs. Eddy to seek to deprive the sick of the help of any science except Christian Science
THE RAILS OF REALITY - By G. K. Chesterton From the column Our Note Book The Illustrated London News January 26, 1918
So, there are probably more instances of GKC's arguments against Christian Science, but this probably suffices. He was not a fan.