1 word: PROGESTERONE. a female hormone. deca in the body turns into that.
progesterone is one of the most confusing and mis-understood hormones.. most guys that run deca (even guys who have been juicing for decades) have no idea what it is or how to combat it. hence they run into trouble.
the reason experienced guys like me don't like deca is because it has to be run for a while since it is a slow builder. so if you like to keep cycles short and recovery fast it is a bad choice. hence I have no desire to ever touch it again. at the very least go with NPP. stacking deca + test together and your progesterone and estrogen will go sky high, meaning difficulty recovering.
i would say there are better stacks to build mass out there that aren't so much trouble. if you do choose deca make sure you know what the heck you are doing ! overall deca has less sides than other steroids believe it or not, assuming you are controlling progesterone and estrogen on it. otherwise it can be an awful experience, just do a search of all the deca horror stories online
most guys who run deca don't run any ancillaries at all and run a nolva pct (ignorance is bliss) so its built up an awful reputation for that reason