Safety concerns are related to the overall idea that diluting medications at the point of care complicates the process of medication administration, which leads to errors.
The safety concerns almost invariably involve other co-existing nursing errors/unsafe practices, such as not labeling a syringe that has medication in it, for example.
Probably best if I reserve the rest of my comments and you read source of the information:
The document is worth reading in its entirety, but the section most germane to this particular discussion is the section on Acquisition and Distribution of Adult IV Push Medications
. ISMP has other statements/publications related to the general topic as well that will come up in a basic search.