Do you mean the online previewer or Kindle Previewer software? Both are pretty good but do not show how your book will look on the full range of Kindle apps and devices. That often does not matter as long as you are not doing anything tricky with your formatting.
The Kindle publishing platform ignores many things that are considered errors in EPUB, but also has its own restrictions beyond what EPUB allows. The number of errors is not significant. The exact nature of each error might be.
This question would fit better in the
Kindle Formats forum.