I can't speak to who you've talked to, but the viability list we have on forums is currently outdated. In general the list gets more accurate the higher your ELO is. If you are not consistently playing ladder matches in the 1300-1500+ rating range you are way more likely to encounter "off-meta" pokemon and movesets. This could be part of your problem. Once you get out of ELO hell the resources become a lot more valid and helpful.
To speak more on rotom and its movesets: simply put nu players value rotom-mow for its ability to compress multiple roles in one slot. it's a pivot with volt switch that can punish ground types in a way other electrics (bar h-trode) cannot. it is relatively fast and can boost its spatk with np, can also fit terablast without dropping a "necessary" move. choice scarf lets it outpace the majority of unboosted threats in the meta and a number of boosted ones as well. i haven't seen specs this gen, but you could theoretically run it i guess. its immune to sandaconda's glare and STAB eq - Sandaconda was on 75% of teams in NUWC - these two things alone make it deserving of a spot on several teams.
For the current meta its primary checks are shaymin, goodra, brute bonnet, decidueye, arboliva, other rotom-mow, appletun, cacturne. these will do better and worse depending on the rotom-set you face but are generally reliable unless you get hit with supereffective terablast. offensively you have a bunch of options vs scarf sets locked into electric or grass moves with the plethora of pokemon we have that resist those types (i.e. jolteon, tauros-paldea-fire, scyther, salazzle, toxicroak, oricorio, vivillon, slowbro-g, eelektross, etc.) as well as using naturally fast and strong pokemon like bruxish.
Here's two teams I built for NU World Cup, I didn't really build with the intention of using Rotom-mow but it naturally found its way on to both teams in two different roles.
https://pokepast.es/38d1cdd25dead657
On this team I needed something fast enough to outrun unboosted attackers and threaten them with leaf storm or volt to preserve momentum. It also provides some nice resistances and lets me pivot into glare sandaconda when it would otherwise kinda eat this team alive. Having the one-time check to fat walls and boosters is nice with trick, for example you can theoretically ward off an oricorio sweep with proper play by aggressively double switching to rotom mow or lock a jolteon/salazzle into tbolt/terablast or sludge/fire move to make it easier to manage.
https://pokepast.es/9e3beb1c60f63115
this team needs the speed from rotom-mow a little less with scarf tauros fire, but at preview it looks ambiguous and could be interpreted as scarf rotom with bulk up tauros so theres some added benefit there. here the role im asking rotom to fill is chipping tox+tect goodra into ohko range +2 mismagius shadow ball/dkiss (somewhere around 75%). rotom uniquely invites in goodra with its stab combination and gives me a pretty reliable will o wisp to cancel lefties recovery, volt switch on the following turn nets another 8% or so, which could be 16 in situations where u np first to get a little more damage. again rotom is also providing nice resistances and the pivot into conda that is the backbone of offense comps.