1. Adrenaline: In order to pull off the spike you will first need to chase down the enemy and charge it up, by which point I think they will know you want to kill them and will be well protected.

2. Speed Reduction: Although SP lowers the enemies speed by double what flail does to you, it is still NEVER good for an assassin to be standing around that slow. A single hex removal and the enemy kites away and your done.
3. Other IAS are much better suited:
--- 3a. Flurry: Only reduces BASE dagger damage, and only by about 2-4. Other than that it has no downsides.
--- 3b. Tiger Stance: Same recharge as SP, if you miss an attack the chain is done anyway, so it has no downsides.
--- 3c. Frenzy: ... Just kidding! Other than when finding the best way to get yourself killed, this should never be put on an assassin.
Any questions? Flail is for tanking with dolyak in areas where you won't be moving at all anyway.