The 6v golf cart Deka Intimidators on my workshop floor claim to have only an 18 month warranty.
They have a 5/17 date code on them, but were treated pretty well, and were free, and still perform well enough that I am going to ut them to work very soon, cycling deeply often.
AGMS in deep cycle duty, really need to be truly fully charged often, or their longevity is considerably poorer than flooded batteries in the same exact duty. The experienced RV'ers and marine guys who know DC electrical, know this well.
I believe this also translates to Starter battery only type of service, but to some unknown lesser degree.
While AGMS have lesser self discharge, and likely retain higher CCA when depleted the same amount as a flooded battery of the same size group , they are not immune to chronic undercharging, which is very detrimental to a battery.
With all lead acid batteries, it is wise to insure they are regularly top charged to the best of one's charger's ability.
AGM's if anything are less resilient in a chronic undercharging regime, than their cheaper flooded counterparts
But, they can charge faster at the voltages allowed by the vehicle's voltage regulator.
So it could be a wash, and it is certainly extremely variable as to the specific vehicle and how the battery was treated within it..
How close to 100% fully charged any lead acid battery is kept, has far more effect on its lifespan, than who built it.
Reports of longevity, good or bad, without knowing how the battery was treated, are pretty much meaningless, as one can take an Odyssey, drain it to 9.8v over 8 months of sitting with a smal lparasitic draw, and it will struggle to provide 1/5th of its original capacity once it is recharged. A valuepower cheapo as battery can be built battery, kept 98%+ charged, can last 5 years.
Listening to a good stereo at a good volume for 30 minutes can take a battery from 98% to 75%.
Getting a battery from 80% to 100% takes no less than 3.5 hours, and that assumes mid 14 volts is held that entire 3.5 hours AND that the battery is still relatively healthy.
Less healthy batteries and lower voltages applied, can increase the time to reach full charge exponentially, even if you recently polished your '280 amp' alternator.
So how was the 'junk' battery treated?
The 6+ year old 'still going strong battery', how was it treated?
Hot ambient temps, slowly drawing it dead, and not truly fully recharging it promptly thereafter, keeping it in the 80 to 90% charged range, is a kick to the groin of any battery, no matter whether it is a 430$ Odyssey, or a 50$ Valuepower.
AGMs are not super batteries, immune to abuse.
They can be little petulant princesses that take their ball and go home prematurely, when not treated well.
When treated well, they can give impressive performance for a highly respectable period of time.
My last group 27 Northstar AGM lasted 6 years, which is not all that impressive, but it also got deep cycled ~1200 times in those 6 years while also being a starting battery, which is dang impressive.
It lived outside the engine compartment, and was promptly returned to a true full charge, often.