In my opinion, this is entirely untrue for Mega Venusaur. Mega Venusaur's flaws(strong Flying- and Psychic-types) annihilate Mega Venusaur with only an iota of prior damage before it even has a chance to do anything, it just dies, it's dead, done, end of story. I'm not questioning that it's an incredible pokemon and equally incredibly dificult to take down but it's just not that bulky and just not that powerful to mitigate its weaknesses. It may be S material, but if so I think it's a shaky S. I'd really appreciate some thorough explaining on how it absolutely is an S rank. (and yes I have read Fuzznip's post)
Your reading too much into those words. Your exact comments can be made about Genesect or MLucario, which are without any doubt S tier. If things didn't have substantial weaknesses, they'd end up in ubers like how MGengar/MKhan. You could have argued Khan was literally destroyed by fighting moves, but that didn't stop it from being a complete monster. Many pokemon could easily outspeed and OHKO with a powerful CC and it had no way of mitigating that, but that never stopped it from being an absolute powerhouse. That was because it was strong, and bulky. MLucario, one who's fast and strong, is killed
easily by any fire/fighting/ground attack, and he can be outsped. Can Mluke mitigate its weakness against Lando-T? No. Not even at +2 in attack and needs +2 in special to take him out with Flash cannon. Does that make Lucario any less of a threat in general? Absolutely not. Can Genesect handle heatran? No, but still doesn't mean it doesn't deserve to be S rank. Same is applied to MV.
Psychic is practically non-existent, only notable users being Reuniclus, Alakazam, Deoxys, and Slowbro. Flying type is the same thing, only notable users are Talon, CharY, Tornadus, and Togekiss. Compare that to the dozens of users who carry fire/fighting attacks Lucario has to deal with.
Here's some calcs to back the bulk of this thing:
Adamant Venusar 252 HP/252 Atk/4 def
252 SpA Mega Alakazam Psychic vs. 252 HP / 0 SpD Mega Venusaur: 314-372 (86.2 - 102.1%) -- 12.5% chance to OHKO
252 SpA Mega Alakazam Psyshock vs. 252 HP / 0 Def Mega Venusaur: 272-324 (74.7 - 89%) -- guaranteed 2HKO
252+ Atk Mega Venusaur Power Whip vs. 4 HP / 0 Def Mega Alakazam: 255-301 (101.1 - 119.4%) -- guaranteed OHKO
252+ Atk Mega Venusaur Power Whip vs. 252 HP / 252+ Def Slowbro: 242-288 (61.4 - 73%) -- guaranteed 2HKO after Leftovers recovery
4 SpA Slowbro Psychic vs. 252 HP / 0 SpD Mega Venusaur: 168-198 (46.1 - 54.3%) -- 49.2% chance to 2HKO
Calm Venusaur 252 HP/252 SpA/4 SpD
252 SpA Mega Venusaur Giga Drain vs. 252 HP / 0 SpD Slowbro: 284-336 (72 - 85.2%) -- guaranteed 2HKO after Leftovers recovery
4 SpA Slowbro Psychic vs. 252 HP / 4+ SpD Mega Venusaur: 152-180 (41.7 - 49.4%) -- guaranteed 3HKO
4 SpA Togekiss Air Slash vs. 252 HP / 0+ SpD Mega Venusaur: 150-176 (41.2 - 48.3%) -- guaranteed 3HKO
252 SpA Mega Venusaur Sludge Bomb vs. 252 HP / 252+ SpD Togekiss: 186-218 (49.7 - 58.2%) -- 62.1% chance to 2HKO after Leftovers recovery
252 SpA Life Orb Tornadus-T Hurricane vs. 252 HP / 4+ SpD Mega Venusaur: 322-382 (88.4 - 104.9%) -- 31.3% chance to OHKO
252 SpA Mega Venusaur Sludge Bomb vs. 0 HP / 0 SpD Tornadus-T: 154-183 (51.5 - 61.2%) -- guaranteed 2HKO
Bold Venusaur 252 HP/172 Def/84 SpA
252+ Atk Choice Band Talonflame Brave Bird vs. 252 HP / 172+ Def Mega Venusaur: 168-198 (46.1 - 54.3%) -- 49.2% chance to 2HKO
84 SpA Mega Venusaur Hidden Power Rock vs. 4 HP / 0 SpD Talonflame: 300-356 (100.6 - 119.4%) -- guaranteed OHKO
The only real monster it can't handle is Tornadus's (either variant). Remind you these are against things you shouldn't stay in on anyway. Regardless of what's deemed S class, some pokemon simply can't handle every single pokemon, which is why you have a team and why those specific pokemon aren't banned. Sorry if too much text, but the whole idea of "a pokemon must handle all situations regardless to be S rank" irks me ^^'