Whirlwind Attack instead of Extra Attack??

[Lalliman]

How reasonable would it be to have a class that gains the Hunter's Whirlwind Attack at 5th level, in place of Extra Attack?

The way I see it:
+ It deals more damage than Extra Attack if there are three or more targets. Massively more in the rare chance that you're fighting a dense formation.
- It's slightly inferior to Extra Attack if there are two targets. Same damage output, but it must be spread out.
- It's obviously worse than Extra Attack if there is only one target.
- It doesn't work with ranged attacks at all.

It sounds like a reasonable trade to me, but are there any factors I'm overlooking?

[Amnoriath]

How reasonable would it be to have a class that gains the Hunter's Whirlwind Attack at 5th level, in place of Extra Attack?

The way I see it:
+ It deals more damage than Extra Attack if there are three or more targets. Massively more in the rare chance that you're fighting a dense formation.
- It's slightly inferior to Extra Attack if there are two targets. Same damage output, but it must be spread out.
- It's obviously worse than Extra Attack if there is only one target.
- It doesn't work with ranged attacks at all.

It sounds like a reasonable trade to me, but are there any factors I'm overlooking?
The main question I need to ask is what is this for? The main question you need to ask yourself is what else is there that will make Whirlwind Attack better?

[Vogie]

How reasonable would it be to have a class that gains the Hunter's Whirlwind Attack at 5th level, in place of Extra Attack?

The way I see it:
+ It deals more damage than Extra Attack if there are three or more targets. Massively more in the rare chance that you're fighting a dense formation.
- It's slightly inferior to Extra Attack if there are two targets. Same damage output, but it must be spread out.
- It's obviously worse than Extra Attack if there is only one target.
- It doesn't work with ranged attacks at all.

It sounds like a reasonable trade to me, but are there any factors I'm overlooking?

I could see it... almost like a Cleaving attack.

You could do something similar while allowing range by creating a variant on the wording from Hunter Ranger's Horde Breaker ability instead.