This is not a click bait – the SUM() function in DAX is indeed broken in my opinion. Just try to add a calculated column with a SUM(…) of another column in the create column formula and then do the same but with a CALCULATE(SUM(…)) and you will see that these will produce two absolutely different results. Luckily, SUM() works as expected when used in measures. However, as you start using SUM() in conjunction with functions like ADDCOLUMNS(), GENERATE(), SUMMARIZE(), etc., you are likely to get yourself in a situation when you calculation produces unexpected results and the culprit will be this inconsistency in SUM() function behavior.
Unfortunately, SUM() is not the only function that can bite you in the butt, many other functions can too. I go into more details in my video and then walk you through the solutions on how to never be stuck with having to troubleshoot this issue in the future.