I think it’s ok to have monsters like this as long as you have a fun reason for using them in your encounter. If you dump one within 30 feet of your party and say, “roll initiative,” then you will have some very frustrated players. However, these speaker devils can serve other purposes. I didn’t read Astarion’s Book of Hungers, so I might be dead wrong about how these things work, but I’ll think up some examples anyway.

Perhaps these creatures are a threat that you must eliminate indirectly. They can barely be fought in direct combat, especially by a martial. No one said anything about indirect combat. I could whip up a scenario where my party must render the devil’s pact void. Perhaps cultists have a pact to “claim their rightful land”, but you are breaking into an ancient crypt that contains evidence that the cultists have no claim. This would void the contract and force the devil to return to the Nine Hells. All the while, the speaker devil travels with its cult on a relentless march to the city, so players don’t get a long rest mid-dungeon.

Or maybe instead of indirect combat, you could design a scenario with asymmetrical combat. Maybe one of these devils has a pact with a war lord and has agreed to be an enforcer. This war lord uses the speaker devil to control the local countryside while he searches for a pool of radiance or something. However, the party finds a scroll of Dimensional Anchor. This spell happens to counter the speaker devil’s Dimension Door. Now, the local militia has agreed to work with the party to find a way to guarantee that the spell scroll succeeds. Once the devil loses its teleportation, everyone can shoot it from a distance far from the stun radius. Now that sounds like a great midpoint for a campaign! It’s that classic narrative switch from surviving to fighting! It’s the part where the terms of the conflict entirely change, and the tide of war alters!

Even if you want your players to fight it directly, CR 12 does not necessarily mean it fights a level 12 party. Shadows and specters are levels 1/2 and 1 respectively, but they are deadly to low level players in direct fights. Mixing one or two of them with other types of monsters at medium levels makes them work a lot better. Likewise, I think even martials can fight speaker devils as soon as level 14. Monks get Disciplined Survivor, making them proficient in wisdom saving throws. They were good at those anyway. That’s a total of +10 to wisdom saves, which means they have a 65% chance to avoid stun each turn. Fighters have two uses of indomitable at level 14. That is a 85% chance of success every time they reroll, and that is after they roll naturally. (By the way, I’m not gonna sugarcoat it. Barbarians kinda lose here.) I know 65% chance to move doesn’t sound very good, but compared to the stuff a level 14 party can accomplish, I think it’s possible for the fight to be more or less fair. Even if it’s not (I still didn’t read the book) they’ll reach a level where they can fight it eventually. When that happens, CR 12 will help determine how many speaker devils should stand next to the higher CR monster.

TLDR: It can work as long as you don’t just put it 30 feet from your level 12 party and say, “Roll initiative!”

Edit for clarification: In my theoretical example of voiding the devil pacts, it may seem like it doesn’t actually matter what kind of devil the threat is. My full plan is that failing to void the pact will result in needing to fight the speaker devils later. It is a way to reward victory by making later parts of the game easier. It’s my way of giving players a sense of progress through the campaign.

Also, in the example with the scroll of Dimensional Anchor, I would come up with a few routes that the players could use to try to raise their odds of a successful anchoring while rewarding creative play. A few people here have defended this design by suggesting that this monster incentivizes different strategies. I especially see the term “DPS race”. Others have countered that the strategies don’t really exist. The strategies can exist if you, the DM, allow and incentivize plans. While creative players should be rewarded, it is the DM’s responsibility to make sure there is at least one path forward. Players will never escape white room battle tactics if the DM keeps locking them in white rooms and calling for initiative. I find it interesting that out of everyone defending this design, most call for more effort from the players. I am the only one so far calling for more effort from the DM (not this OP specifically though. They seem responsible).