The source – alexeytrudov.com
The official Google support says that copying content to other sites is a good reason to use rel = canonical, but the canonical url and its copy can be on different domains. Google also said that canonical is almost as strong a signal as a 301 redirect.
But when it is necessary to merge similar pages, the gluing does not always go smoothly. Therefore, the effectiveness of the default cross-domain canonical is questionable. Alexey's research confirms the following conclusion: cross-domain canonical does not always work. This is a weaker signal than it might seem when reading Google Help.
According to Alexei, it is obvious: if canonical worked quickly and clearly, a different picture would have been expected. The sample is large enough to smooth out accidents like the physical disappearance of the original. “It looks like canonical works every other time here,” writes the author of the study.
You can read more about the research on the author's personal blog.