Sleepless im not sure if i agree that grass is a bad typing. Immune to spore and resists to grass and water is good. If the tier is so heavily bulky water (only suicune and swampert are the only ones coming to mind that are top UU threats, unless im missing something?), thrn wouldnt a grass type be that much better, and not disadvantageous?
But, theres a difference betweeen versatile and picking what walls a mon. Serp is of the latter, where hp determines what it cant bust through, wbich a teammate will have to take care of. Knowing what walls you is a huge advantage in teambuilding and gives you great oversight.
Now, versatile is a mon that has that unpredictability aspect, all of which are viable. Victini was both a nuke with few switch ins and versatile due to its fantastic movepool and ability to go mixed with its high bp coverage (thunder, GK, focus blast, bolt strike). You literally could not switch in unless you had phys def mega amphy.
Glare is a great spammable option on serp, but are status moves broken because they allow a check to be incapacitated? If amoongus puts uses spore on a cleric not named blissey, then spreading status team wide (burn, para, etc..) combined with taunt pressure doesnt allow the opponent to recover. Granted amoongus wont be incapacitating and sweeping, but the principle is the same. Glare has good coverage with only electric types being immune, but statusing a check (crobat since everybody keeps bringing it up) it doesnt even necessarily mean insta win. Not even remotely sincd serp is forced out fearing the BB.
The key difference from Victini and Serp is the force out. After VC, tini was very vulnerable due to the def drops and the switch in could pursuit trap, but this didnt deter tini from nuking left and right and the pursuit user could only come in after something was KOd. Serp is forced out literally because it cannot bust through crobat unboosted. Same thing for amoongus, roserade and AV entei, some of the best defensive checks. Offenzive checks can come in after the KO; scarf darm, ape, crobat (again), entei (again), noivern, mega pidgdot, mega beedrill, mega aero, etc..glare and boosted LS hurts the offensive checks, but they should almost never be switched into serp. But, tbats no different than other sweepers or wallbreakers (cb hera, sd ape, np azelf, nidos, curselax, etc...