It is very common for editors to retime shots in the Avid, and Avid has clever 'flow motion' tools to give a great result in low quality DNX36 (the most common codec using in long-form editing). You should always use the Motion Effect tool as shown in the screenshot below.

In general it is safe to speed up the footage by 200% or 300% (drop every other frame, drop every second and third frame). Slowing down though becomes more problematic, even if the material has been shot at 50fps and you want to speed up to 25fps. While the maths is correct for a 200% speed up, due to the shutter angle, the resulting output may have a 'stuttering' visual aesthetic.

More problematic is speed ramps or retimes using Flow Motion in Avid. Achieving good retime results in uncompressed 4K images without softening or warping can be extremely difficult, and clients can be disappointed with the results (as can broadcasters at Quality Control stage).

We have the best retime tools currently available in the world, but even so, careful consideration should be used when utilising any flow motion retimes or ramps in your timeline. If at all possible, do not use them unless you are aware of the potential image consequences.