But what exactly do you mean by "stake"? For example, if I controlled 51% of the stake in the genesis block, could I take over the network? If the answer is no, then the statement "Bitshares is secure provided 51% of stake is honest" is incomplete.
Any stake older than 101 blocks before the current block is irrelevant in bitshares, just as any work performed prior to 6 blocks before the current block is irrelevant in bitcoin, as a general guideline.
With PoW, it's very easy to define what we mean by "work," because work relates to a physical quantity.
Work is defined by hash rate. Stake is defined by number of units of the native currency. Both are quantifiable.
Number of units of native currency you have when, exactly? You could gather a large stake, spend your coins, then go back to when you had the large stake and work from there again.