From the UBC Centre for Teaching, Learning, and Technology Edubytes is this useful discussion of OER in STEM disciplines
These include the need for complex diagrams and illustrations, the costs reusing no-cost resources, the need for more OERs beyond first year courses, and the need to have access to not only the published OER but source files within (and example of an image that needed to be rebuilt).
In summary, there are some great STEM OER options available, but they tend to be limited to introductory courses. Broadening the availability of open resources for program-required STEM courses will entail larger investments compared to what is sufficient in non-STEM courses. However, if we invest in creating good quality STEM OERs, widespread adoption is realistically achievable — providing wide-ranging benefits to students and instructors alike.