We don't even know how to define ch words and sh "words". We don't know what constitutes a "word" in Voynichese or if there are any in the linguistic sense.
Just because two letters are adjacent doesn't mean they have the same function.
For example, consider the English words "morph" and "mophead" (mop-head). The "ph" in the first one is not interpreted or parsed the same as the "ph" in the second one, even though the letters m o p and h are in both words and are in the same order. Even though "ph" and "ph" look they same, they have different affiliations with the other letters and they are pronounced differently, as well.
Just because two letters are adjacent doesn't mean they have the same function.
For example, consider the English words "morph" and "mophead" (mop-head). The "ph" in the first one is not interpreted or parsed the same as the "ph" in the second one, even though the letters m o p and h are in both words and are in the same order. Even though "ph" and "ph" look they same, they have different affiliations with the other letters and they are pronounced differently, as well.