The basic theoretical mechanism of mass fluctuation has never AFAIK been professionally critiqued such as to disprove its validity.
The main issues with The Woodward Effect, both theoretical and experimental, are discussed here.
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There exist no positive experimental results that have been widely replicated; indeed, all experiments independently performed outside Woodward's own lab have yielded null, or questionable results. The Database section at the above site is in the process of cataloging these (there are many more as yet unlisted).
The implication of the existence of mass fluctuation means that it's possible to build a locally over-unity device, although globally conservation is theorised to obtain via the proposed Wheeler-Feynman transactional mechanism. The local over-unity property can be appreciated by noting that a device operating at constant power consumes energy linearly with time, yet, for a (theoretically predicted) constant thrust from such a device, constant acceleration would obtain, and thus the kinetic energy produced would be quadratic with time. Thus after some characteristic time T, more energy is continuously available than was input (seen only locally).