Do the Bronze Scout's burrow speed and Earth Armor trait make it stronger than a CR 1 creature?

Question

The Bronze Scout Clockwork construct from Mordenkainen's Tome of Foes (p. 125) is a CR1 monster, but has a set of traits that seem to make it incredibly powerful, unless I'm interpreting them incorrectly.

Speed 30 ft., burrow 30 ft.

Earth Armor. The bronze scout doesn’t provoke opportunity attacks when it burrows.

Based on the basic idea that players and monsters can move and attack in any combination (as in, move 15ft, attack, move the remaining 15ft), what is stopping the Bronze Scout from burrowing 15ft up to the surface to attack a PC, then burrowing down 15ft after, which according to Earth Armor, would not trigger an Opportunity Attack?

If I'm reading this correctly, how could the PC's counter attack or stop the Scout from re-burrowing on its turn? My first thought was that the PCs could set up their reaction to attack it, but since you cannot move AND attack on a Readied Action, the PCs would have to hope that the construct would attack one of them that other allies were adjacent to.

"When you use Ready, you prepare an action or a move, not both." - Jeremy Crawford

Finally, the flavor description of the Scout (on p. 124) indicates that it can see above ground when burrowing:

A bronze scout seldom emerges from below ground; thanks to its telescoping eyestalks, it can observe enemies at close range while most of its segmented, wormlike body remains buried.

Its got an INT of 3 and a WIS of 14, so I feel that it should be able to make good attack decisions on its own, such as selecting a target to pop out and attack whom doesn't have a squad of allies standing beside it ready to counter attack.

Finally, the creature hits with a melee attack of 1d4+3 plus 1d6 lightning damage, and it has a once-per-rest area lighting attack that does 4d6, so its seems kind of lethal for a pack of 1st level characters.

Am I reading these rules incorrectly or is this monster much stronger tactically than a CR1 would suggest?

Answer 1

It is CR 1

The Dungeon Master's Workshop in the Dungeon Master's Guide has some good advice for calculating CR. You essentially average the monsters Defensive capabilities with its Offensive capabilities.

Defensive CR:

• The monster's 18 HP is modified by its resistances to result in a DCR 1/4.
• This means the monster should have 13 AC, but the Magic Resistance trait modifies the monster's effective AC to 15, bumping it to DCR 1/2.

Offensive CR:

• The monster's basic attack deals only 8 damage per round (DPR) on average (on a hit). The once per encounter feature deals about 35 points resulting in an added 10-12 damage per round averaging to 9-10 DPR. This places it squarely on OCR 1.
• With a +5 attack bonus, the monster hits more often than a CR 1 boosting the monster to OCR 2. (The save DC for Lightning Flare is comparable to CR 1 creatures and doesn't affect the OCR).

Overall CR: Taking the average of 1/2 and 2 CR we get 1.25 which rounds down to 1 CR.

But that's not the whole picture

Tactically speaking many monsters break the rules of Challenge Rating. Challenge Rating is designed to account for damage output, survivability, and some unique traits. Tactical strengths can make low CR monsters shoot above their power and tactical weaknesses can make high CR monsters easier to deal with. The latter of which is often the crux of what makes a PC party strong, their cooperation and tactics.

Solutions

There are a number of ways to fix this problem, but here are three that I have used to great effect.

1. Treat it like a higher CR. The Flyby trait essentially functions very similar to Earth Armor which I have found does adjust survivability by a bit. Usually not enough to raise it by a full Defensive CR rating, but sometimes enough to tip a monster into a 0.25 higher Average CR (which would call the Bronze Scout a CR 2, albeit on the weak end of these).
2. Don't use that tactic. Not every monster has to employ perfect tactics. Perhaps the Bronze Scout only utilizes the Earth Armor trait when its time to run away or reposition to attack another target. This means the PCs will not have to worry about it burrowing away and have the chance to attack.
3. Tell the PCs in advance. Players can be very crafty when they are told ahead of time about a particular tactic the enemy will employ (either by an NPC or by witnessing a combat where the creature is involved). Even if they aren't already aware, after the first round of combat they will probably begin to adapt by using tactics such as employing the Ready action (to name but one option). Just remember the usefulness of "show don't tell" when possible with this method.

Answer 2

Yeah, it's tough, but it ain't that tough.

Some other 1 CR creatures: bugbears, dire wolf, ghoul, and quasit.

And, sure, the burrow/attack/burrow routine could cause some heartache, and those immunities will suck to deal with, but other CR 1 monsters got some tricky moves, too.

Some things to keep in mind:

It's a runner, not a fighter

It's a scout. The description says:

A bronze scout seldom emerges from below ground; thanks to its telescoping eyestalks, it can observe enemies at close range while most of its segmented, wormlike body remains buried. If it is detected, the bronze scout deters pursuers by sending electrical shocks through the ground while it retreats to safety.

That suggests that if it can, it runs.

The Bronze Scout does not inherently have a reason to attack the PCs

Per the book, it has no inherent reason to attack -- no hatred of the living, no need for food, no territoriality, no malevolence.

Sure, it could be programmed to attack (one could imagine), but that's beyond the definition in MToF.

It gets 1 Lightning Flare

Its Lightning Flare doesn't recharge until after a short or long rest, effectively meaning it gets it once in an encounter, before it either gets killed or gets away

It has inherent flaws

All constructs get a clockwork enhancement and a clockwork malfunction. For instance, some of them are vulnerable to overheating, and will become incapacitated for a turn on a 1 out of 6. That's a serious weakness.

On the other hand

An evil gnomish mastermind could have a hoard of them, all programmed to attack and run, attack and run. That could suck really quick. But that's beyond the book description.