It may be a mix of support-hate and underestimation. I've been a long time Warlock player and I love the guy, but I can't get my finger exactly on why. Although not because of his golems, since the games I often do better with him I never get the agh+refresher combo, and only 1 of them half the time; I prioritize pipe (and/or mek if nobody builds it) over them. And neither because of his upheaval, because despite it being a beast of a slow, I already loved the guy before learning to use it.
His heal/dps is really good and versatile imo, and watching people get really annoyed by fatal bonds gives me my kicks too. Golem is a really fun way to initiate and counter-initiate, and a way of being useful even from behind the grave if you got picked off in the teamfight. However, I didn't use upheaval that much until recently: when I started to play him the cast range forced warlock to stand just at the fringe of the area, and it put you in a very open position despite being one of the best slows in the game. It has been buffed since then and now the cast range is considerably larger, but it still felt scary to me to open myself for such a long time when I could be hitting with autoattacks (which tbh don't do that much) so I did the old popular build of stats instead of upheaval. "It's too situational", I lied to myself.
People definitely underestimate it like I did. The usefulness you gain from upheaval lvl 1 to lvl 2 is insane (half the time of channeling needed for full slow), and the linger time (after they get out of the area or you cut the spell) is the most indecent out of all de/buff linger times. And positioning isn't that hard with such a cast range. HOWEVER: you're useless by yourself now. If you gank you must do it on a lane whith someone who can hit hard for the duration of your upheaval, or along with someone like that, or wait for someone else to gank to use it. The upside is that If you position it correctly, even if they try to run at you instead of away they will have a hard time and your carry can pick them off easily.
The idea is to be far enough from them so they can't reach you in time, but near enough so they can taste the slow for a few solid seconds before getting into linger time. And obviously don't use it if they have disables up & loaded.
Slahser's way unsurprisingly has a warlock video about maxing upheaval first. It's a great example of what it can offer. You don't have to play it so aggressively to make it work though. Just use it. Put 2 levels in it as fast as you can at least, and use it whenever you think you can pull one off.
Bonus: if you use CTRL for global orders, I think you can order the golems around without disrupting the upheaval channel (Warlock will ignore you unless you order him directly).