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Your shower, if properly constructed, could have a moisture barrier behind the CBU (or possibley mud?) walls or it could have a direct bonded waterproofing membrane on the inside face of the wallboard. If the first, you wouldn't damage the waterproofing with aggressive grout removal, if the second you need to be much more careful.
If you have no need to worry about the waterproofing, a utility knife can be effective in removing un-sanded grout from narrow joints.
My opinion; worth price charged.
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