The quality of the hop-unit is as important as the bucking imo, a cheap unit will have flex in the parts, or inconsistencies in the finish which will alter air seal at best and inconsistently apply hop-up at worst. I've often said that there's no such thing as an accurate airsoft gun, just a consistent one. The key to finding that consistency is removing all of the variables;
hop-arm flex applying inconsistent pressure on the rubber,
hop rotation (along with the barrel) caused by an off-spec hop unit not fitting snugly in the outer barrel,
a loose adjustment dial moving around with the vibration of the gearbox,
off-spec chamber leaking air because the bucking doesn't fill it properly, or worse being too tight and deforming the bucking resulting in mis feeds and jams...
I could go on!
Short version, decent hop-up unit is key to performance. One of the many reasons Marui am bestest... they make the best hop-up units going.