That's true. However, the pokemon's flaws don't start and end with guts. Objectively it's a good ability and if it gets the chance to improve the speed, it could sweep. To do so, it needs better moves, since the stats are fixed among eevees.
Now, I'll return to Leafeon.
If I were to give it an ability I would go either to Technician (since it gets Bullet Seed, Quick Attack, Aerial Ace, and Bite) which can be boosted. Alternatively I would go to Thick Fat to make it more defensive. Problem is, the first choice makes it akin to Breloom, the second resembles M-Venu and we don't need another defensive grass type.
Another option I thought of is Triage, which makes use of Synthesis when needed, however that's really limited and Leafeon can't even rely on Giga Drain offensively.
Last, I thought of an offensive Unaware user which Swords Dances up and disregards all defensive boosts.
Do you think any of those work?