We recently went from 8.54 to 8.55 and the big difference is to be the Fluid UI changes. Fluid UI is the new standard in 8.55+ peopletools and classic is optional. If your application is in classic mode then you've to analyze the tools changes carefully. Also, we had issues when transferring from iHub to content applications. Our iHub was running on PT8.54 and content application was upgraded to 8.55.
We upgraded from 8.54 to 8.55.14, and we did not see any major challenges. But for Elastic Search we had to grab a Patchset from 8.56. Elastic search has lots of changes in 8.56 like direct file integration with Elastic Search server instead of integration broker.