Well, here's my view on Tempest:
It needs Shadow Strike + Quickblade to work.
I tried everything, but because quickblade reduces cooldowns it's the only ability that makes me feel like the tempest is interesting. Otherwise, I ran into the same issue every single time. FoI > meh not gonna much. FoL > meh auto-attack / abuse rift stuns. FoF!!! OMG pew pew pew pew funz! Basically feeling kinda meh outside of FoF just didn't feel right.
I saw the tempest as being built around flasks and pumping that killer's alchemy, and then using other skills to supplement your ability to use flasks. So with quickblade I usually had 2 flasks ready to go which gave me options during combat. But every build I tried without it just left me wanting. It's a shame because some of the best abilities to use with FoF are throwing knives and spinning blades with poison weapons active. But the moment FoF ends, you have nothing else.
And so I stayed with Quickblade just so I could always have abilities on the ready. It's the only build that makes me feel like a storm rampaging across the battlefield.
Assassin let's you have your big opener and then you can supplement your build with other abilities so you're always using something. I dunno, I guess tempest just feels like it needs another 10 ability points and another hotbar.
Which brings me to my next beef about tempest, it's pretty crap until you have around 15 ability points. The tempest tree just requires a heavy investment to work to the fullest. Again I tried every combination of early tempest tree builds, taking just 1 or 2 flasks and while I was able to hobble along with just part of the tree, it's a spec that doesn't truly shine until you have all your tools. So I pretty much felt like playing assassin would have been the better choice, because by the time you're really a force to be reckoned with, you've reached the end game and the assassin tree would have been a powerful class to play as that entire time.
I mean, just look at the assassin tree, 6 points down the left side and you're pretty much golden, you can spend the rest of your points in other trees. Tempest on the other hand, 6 points down the right and you're missing a flask, thousand cuts, and ride the storm. Go the other way and you're missing Killer's Alchemy and Flask of Fire. So go 5 down the right and 1 on the left, you miss Ride the Storm and quicksilver and thousand cuts. An 8 point investment down the left + FoF and UF gets you the bulk of what you need, but you're still missing either Thousand Cuts or Quicksilver and you're missing Killer's alchemy. It boils down to the fact that Ride the Storm and Killer's Alchemy are so far down and they really make the spec.
At the end of the day, play what you enjoy. Of note, I don't use or count the FoF + Thousand Cuts in my tempest playstyle, pretty sure it's not meant to work. If it's intended, that pretty much changes everything.
Figured I would add my build: Rogue - Tempest
I know some people have reported trouble hitting with Shadow Strike/Quickblade - but it's just about positioning which is easy to do when things are frozen/in stasis. Also flank attack puts you in prime Shadow Strike range.