To put it shortly, the reason I think venusaur is bad and overrated is because it gets worn down too quickly by the things it's supposed to check and other bulky grasses are much less passive and provide more utility.
Even though venusaur is decently fast and decently strong, it's still easy to switch into and can't touch bulky offense or balance. Obviously it can hit hard against hyper offense, but it gets overpressured way too quickly. Obviously it checks a lot of mons like kartana, koko, bulu, greninja, keldeo, heatran, zygarde, magearna and mawile but it's bad at checking all of those because hazards, burn, u-turns and volt switches wear it down way too quickly.
Greninja just spikes up on the switch to venusaur as venusaur takes the rocks damage. Greninja then switches out on the double/giga drain and goes into something to force venusaur out. Venusaur then has to come in on spikes+2 rounds of stealth rocks if they were there before and specs dark pulse easily 2hkoes. Tapu Koko just u-turns out into something to force venusaur out and Venusaur takes any hazards+the u turn damage and has to switch out and come back in on hazards the next time it has to check something and it will probably be worn down to 70% and will most likely get 2hkoed. Keldeo, rotom and toxapex can all just fish for a burn on venusaur and toxapex can stay in on venusaur indefinitely. AV bulu just nature's madnesses venusaur and forces it to waste a recover as it goes into something to force venusaur out. Band Bulu just does a ton with megahorn/zen headbutt and sd rockium can blow it away with a little bit of chip. Bandgarde does a ton to venusaur and can stay in on venusaur and force it to recover. Dragon dance just sets up to +2 and kills with groundium. Venusaur can't even break it's sub without giga drain. Heatran does like 50 with magma storm + magma storm damage and shuts down venusaur's recovery. If heatran is spdef, it can just burn with will o wisp or fish with lava plume.
Venusaur is a pretty safe bet vs mawile, kartana and magearna but they all have some ways around it. AV mag just volts out into somehting to force it out, it can beat it if it's sg psychium (which is no set) and regular sg cm can 1v1 if venusaur doesn't have earthquake
or just hax through with freeze and paralysis. Kartana walls venusaur if they don't have hp fire and the rather unconventional sets darkium steelium and normalium can all break through. Mawile can 2hko if it's running iron head which is a bad set.
Venusaur's only recovery is synthesis which gets easily stalled out, it doesn't get too many opportunities to recover up vs. offense and if sand is up, venusaur is completely shut down. Venusaur is also almost always forced to run sludge bomb/hp fire/ eq. Without giga drain, zygarde can wall and it can't ohko ash greninja as it's forced to spam recover and get flinched down or get 3hkoed by dark pulse on the switch and not kill back. Without sludge bomb, grass types wall and lele can come in for free. Without eq/leech seed it can't do anything to magearna and heatran. Without hp fire, it's hazard fodder for ferrothorn and setup fodder for kartana. Venusaur never gets space on it's moveset to run utility moves like knock off and sleep powder.
Another thing is that all the other bulky grasses have more utility and venusaur doesn't provide anything to a team besides "walling certain threats." Other bulky grasses can wall the same things and do more for the team. Ferrothorn has the invaluabe steel typing, punishes physical attackers for using contact moves, can set hazards, leech seed, spread thunder wave, knock people off, is extremely bulky on both sides and it's stabs are still decently strong. AV Bulu can check the infamous lele well as long as it doesn't get psyshocked and doubles as a wallbreaker and it is very hard for offense to switch into. Amoonguss has spore, stun spore, foul play, clear smog and regenerator so it doesn't get worn down so easily like venusaur. AV tangrowth has an insane movepool and great coverage/utility with eq, rock slide, knock off, hp ice, leaf storm, giga drain, hp fire and sludge bomb. Physdef tangrowth can give off helmet damage, spread sleep powder, is extremely bulky in the physical side and still has the busted regenerator ability.
Not to mention, the meta isn't too kind to venusaur either. Spikes are still as good as ever and are the best way of wearing down venusaur. A lot of teams right now can spread burns with will-o-wisp, scald and lava plume and pex is still top 5 in usage and is the best venusaur check in the game. Flying and psychic are still fantastic offensive types. Pinsir and hawlucha are top tier threats and lele, latios, zam, hoopa and medicham are great too. Not to mention, ttar usage is as high as ever and it shuts down venusaur completely. Volt-turn is also a great strategy rn and it destroys venusaur as it gets worn down by u turns and is forced out by a powerful breaker before it can recover.
On stall, you should always rub sableye, scizor or no mega. Sableye checks medicham and zon z heatran and has the invaluable ability magic bounce. Magic bounce bounces status/hazards/taunts which are the best ways of wearing down stall and if you can't keep hazards up you're basically not breaking stall. Not to mention, since it's a mega it can absorb tricks and knock offs to keep chansey's eviolite intact. Sableye also has the great ghost typing so it can avoid being trapped and the dark typing to wall mono-stored power pokemon. Sableye can also knock off mons and spread burns. Running venusaur/scizor forces you to run mew or cresselia to check medicham and neither provide near as much utility as sableye. You can just add an amoonguss or a zapdos to check Kartana on stall. Look at the stall tdk brought in spl. That team has no sableye and is a feast for mega medicham.
Esteemed trophyless user blunder has said on multiple occasions that he doesn't like venusaur and thinks it's bad so it must be true.