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?