#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.

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.

#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.

You stated that you want to keep restrictions on specific attacks such as the one bite attack, two claw attacks of dragons. The reason you want to do this is to reduce complexity, but if you need to supplement your rule with additional rules on when extra attacks are allowed. 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.

#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.