Re: [ANN] Aerium [AERM] POW | POS | HIGH APR | LOW SUPPLY | LOW PREMINE | GPU MINING
If it's pure POS then it is "quite guaranteed". There is variance with POS of course but over time your POS rewards should accurately reflect the amount of coins you hold in relation to all coins actively staking.
it's hard to tell for sure as I keep those coins at Coinsmarkets as I can always get moar or dump them besides they're known to payout the staking thang. Now with the blockchain split it's even harder to tell 
but if you keep them on an exchange you don't get any rewards from staking of course.