Is there a drawback to Flail Snail's Shell defense?

Question

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?

Answer 1

There are no drawbacks! Stay in-shell.

Looking at the Flail Snail entry (Volo's Guide to Monsters, p. 144):

Shell Defense. The flail snail withdraws into its shell, gaining a +4 bonus to AC until it emerges. It can emerge from its shell as a bonus action on its turn.

There is no language in here that states withdrawing into the shell prevents any of its actions. There also is no language in its actions that states they are only available if out of the shell.

Whether or not the intent was to provide a downside, the actual entry does not. There isn't any mechanical reason why it wouldn't always be in its shell for an AC of 20 at all times.

A contrasting example

The Tortle (The Tortle Package, p. 23) uses shell defense as well, but it adds some other bonuses/penalties:

The tortle withdraws into its shell. Until it emerges, it gains a +4 bonus to AC and has advantage on Strength and Constitution saving throws. While in its shell, the tortle is prone, its speed is 0 and can’t increase, it has disadvantage on Dexterity saving throws, it can’t take reactions, and the only action it can take is a bonus action to emerge.

Without the language similar to that of the Tortle above that specifically calls out penalties, we have to read that there are none for the flail snail.

Answer 2

By this argument, the drawback is that the snail is blind.

If the tentacles are all destroyed then the snail retracts into its shell and gains total cover. Therefore (the reasoning goes) when using Shell Defense, the tentacles are still outside the shell.

But what's not outside the shell are the snail's eyes. A creature with total cover can't see through the cover* (and if its eyes were sticking out, it wouldn't have total cover, now would it?). So it should be considered Blinded while inside the shell, with all the usual consequences (most importantly advantage on attacks against it).

I say "considered Blinded" because, while it doesn't strictly have the Blinded condition, its line of sight is blocked in every direction, making the entire world heavily obscured.

A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.

This reasoning relies on the argument that the difference between Shell Defense and the fully retracted state is that the tentacles (and only the tentacles) are outside the shell. I don't completely buy that claim, but on the other hand, it's a snail. When a snail hides inside its shell, it pulls its eyes in.

Flail Snails do have tremorsense, and that will still work, but tremorsense isn't a full replacement for sight.

*unless it's provided by something transparent like a glass window or wall of force spell; that would not seem to apply here.