In this Flail Snail Tactics blog post the author argues that there are no drawbacks to Flail Snail Shell Defense ability.
Shell Defense, as an action, is one-way: it activates a benefit. To deactivate the benefit requires only a bonus action—and why would it ever deactivate the benefit?
Following the principle of always interpreting every fifth-edition rule absolutely literally tells us something counterintuitive in this case: Shell Defense doesn’t prevent the flail snail from attacking. “The flail snail withdraws into its shell” seems like it should prevent it from attacking, but contrast the benefit of Shell Defense, a +4 AC bonus, with the Flail Tentacles trait:
“If all its tentacles die, the snail retracts into its shell, gaining total cover” (emphasis mine).
A +4 bonus to AC isn’t total cover; it’s not even three-quarters cover. Even after the flail snail retracts, something is still sticking out of that shell—and by deduction, it’s the tentacles.
Is this true according to RAW and RAI?