Im afraid the problem is it that it does get taunt.Serperior would actually be much more threatening if it had Taunt, but seeing as it doesn't, without Life Orb it can barely 3hko clefables with spdef, and without knock off it can't properly deal with chansey either, though chansey risks a substitute on a toxic, but seismic toss 3hkos LO. It's also 100% walled by Mega-Alt, unless some random HP Poison variant lol. Spdef Amoonguss is uncommon, but I'm pretty sure amoonguss can take +2 HP Fire and ohko with sludge bomb after LO
Personally here is how i rate current stallbreakers in terms of potential
As in i see Serp as top 5 but there are harder mons to deal with for stall or at least harder mons that i usually struggle to beat.
Amoonguss is a pretty easy hard check to serp and is pretty much the only mons that can deal with serp when it does carry taunt. However taunt/synth/leafstorm/hpfire is ridiculously hard to deal with as once it gets in it outspeeds everything can heal off damage from stoss or moonblast and just blitz through a stall team solo. Luckily nobody really runs that setup but if they do then its nearly a 6-0 from turn 1 or when it gets a clean-switchin.