I've said it before but really feels like a flaw that the halvings are discrete and happen suddenly every four years, instead of gradually each block. As far as I can tell the only advantage to it is that it makes the math simpler. The disadvantage is that it creates weird market dynamics in which large amounts of mining capacity are plunged into unprofitability in one instant. If I wanted to run a 51% attack, I'd look to buy up suddenly-unprofitable capacity immediately after a halving.