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.