There are a lot of questions on "Extra Attack vs Multiattack" and it's become my opinion that the act of separating the two generates more confusion than value as a gameplay and balancing lever.
It seems that the main reason Extra Attack and Multiattack are distinct is because it prevents monsters with Multiattack from utilizing the all-too-common shove/attack sequence that Extra Attack PCs utilize… but seeing as most monsters lack proficiency in Athletics they'll lose the contest almost any time they attempt it, so I don't see how that could be a worthy balance consideration.
I'm considering implementing a house rule that treats Multiattacks as Extra Attacks. Any time that a monster is able to attack multiple times, the attacks are treated as separate attacks granted through the Extra Attack feature. Specific text that limits the number of a particular attack per multiattack is still respected (eg. A young red dragon is given three attacks, two of which can be a claw attack, one of which can be a bite attack).
What would be the balance ramifications of this house rule?
Best Answer
There are quite a few monsters with Multiattacks that are difficult to emulate.
The chimera (MM p. 39) can optionally use a breath weapon instead of another attack.
Some monsters can only use Multiattack for melee but not for ranged attacks, e.g. the cyclops, drow elite warrior...
The glabrezu can exchange some of its attacks for a spell.
The grick can make a second attack if its first attack hits.
The violet fungus makes 1d4 attacks.
The goblin boss can make two attacks, but only in melee and the second attack has disadvantage.
The Nycaloth can teleport as part of its Multiattack.
It seems to be even more complicated with the house rule.
The reason you want to do this is to reduce complexity. But you stated that you want to keep restrictions on specific attacks such as the one bite attack, two claw attacks of dragons which is necessary to not unbalance damage output. But if you need to supplement your rule with additional rules on when extra attacks are allowed, it seems that the change does not fulfill its purpose. Additionally, people know the original rules. Transitioning to new rules will always take time, so you should be sure that the benefit is significant.
Extra attacks and Multiattack affect different domains of the game.
Since the extra attack feature appears on player classes and Multiattack appears on monsters, there is usually not much interaction between the two, except in a few cases like a ranger companion or druid wild shape with fighter levels and similar. Therefore, it seems to be easier to just use the class and monster rules present.