It really depends on how much process control you are looking for. For the most typical single speed motors, that is achieved by turning few fans off during cooler ambient days, but this is considered a big step change. 2-speed motors would provide a little more control, but the VFD will provide the ultimate precise control and provide the most efficiency. Since VFDs are getting cheaper, this option is utilized very often in refineries, LNG plants, etc.
The biggest operational concern is critical speed. Likely, there might 1 or 2 critical speed zones between 0-100% VFD speeds that might have a structural natural frequency coincide and create resonance. During commissioning, these critical speeds must be identified and locked out.