Generally, I have found that Inkscape generated PDFs containing gradients with semi-transparency are fine if viewed and printed with Adobe Reader. I don't use any other PDF viewers for that reason, and have no intention of installing any, so I can't replicate the problem.
All bets are off for third party viewers - such as default viewers/in-browser viewers. And there is nothing you can really do that will make them work properly. Rasterizing gradients is certainly one possible solution.
There are several similar questions here on GDSE that involve missing content in PDFs created using different software (not just Inkscape) which have been resolved by using Adobe Reader/Acrobat Pro.