Are the Truvativ powerspline BBs reliable? When I first got my Haymaker 1500 about 2 years ago the BB crank interface was the Truvativ powerspline pattern.  In the first winter my left arm came loose when it was cold (-15*C or so).  Since it was cold I wasn't really looking at my bike that much and the bolt loosen up and the arm began to wobble.  This led it to damage the interface of the arm.  After using some tin shivs to take up the slack and allot of red threadlocker on it (which I did in my place and gave it a day to warm up first and another to set, and used put more than enough torque on it), and then it was good until 2 months later were the bolt loosen up again (but with only a slightly wobbling of the crank arm) despite all the red threadlocker on it.  Tried to fix it again but the 8mm bolt (old) broke off into the BB with out any real force.  So, I got a new BB and left arm and it was good for for 3 months in the late spring, same thing but with the right arm.  The arms interface was fine (caught it early, so no damage).  I got a new bolt this time with thread locker, 40Nm of torque, and gave it 2 days to set.  It was fine for 5 months when it began to wobble again and this time the bolt broke off into the BB when I was riding.

Since then I replaced the BB interface system with the Octalink and I haven't had a single problem in the last few months.

So are powersplines really this bad, or did I just have a lot of bad luck?
The TruVative power spline BB is a decent, if inexpensive design, and is unlikely to have failed through a design fault. 

All bikes, regardless of price or quality level, require periodic maintenance.

One of the most common issues on a relatively new bike is the crank arms "seating" on the BB spindle, and needing the fixing bolts to be re-torqued.  This will happen on every BB and crank design, with varying time frames between occurrences. 

The Octalink BBs are known for loosing fixing bolts, actually. :) 

This is not "bad luck". But neither is it a design flaw. It is missed maintenance.  They should be checked every few weeks until you get a feel for how often you need to do the maintenance. FYI, every bolt on your bike can have similar issues, and should be re-torqued periodically. 

Unfortunately, as you discovered, a crank arm ridden while loose will damage the crank, and often the BB spindle, and it can be an expensive repair.