If there are Spell or Trap Cards, or face-down monsters, on your opponent's side of the field, "Drillago" cannot attack directly.
If "Drillago" attacks directly, and your opponent activates an effect to reduce one of his face-up monsters to less than 1600 ATK, there is no longer a condition allowing "Drillago" to attack directly, and a replay occurs.
If "Drillago" attacks directly, and your opponent activates "Ready for Intercepting" from his hand, because his/her "Makyura the Destructor" was sent to the Graveyard earlier this turn, to flip one of his face-up monsters face-down, there is no longer a condition allowing "Drillago" to attack directly, and a replay occurs.