Not sure what might be happening on your system.
I am running latest Safari ver 12.1 on Mojave 10.14.1, and DuckDuckGo works with no problem. In fact, I have had DDGo set as my default search engine in Safari for several years, and it never fails for me.
Maybe you have an outdated extension installed in Safari that does not work correctly. Safari DOES struggle with some ad blockers that have not been updated yet (or in some case, won't ever be updated.)
But, if you have other struggles, then there remain a lot of alternate browsers that you can use: Google Chrome, Opera, OmniWeb, Roccat browser, iCab, Firefox developer version. Those, plus a couple others, are what I have on my system, and each gets used regularly for specific purposes. Most used for me, however, is Safari.