Breloom is a classic example of how being very specialised in a role is generally better then those jack-of-all-trades or all-rounders. He has a precise movepool that tailors most of his needs, has great power and excellent abilities that he can utilise well. Let's take a look at how this 460 base stat total pokemon is recognized as one of the most fearsome threats today.
First, its movepool. Of course there's its trademark Spore which instantly neuters a threat, earning its notorious reputation of how hard it is to switch into. Of course if this is all Breloom has, it wouldn't be all that special from the other bulkier Spore users. However, it also has Swords Dance, Bullet Seed, Mach Punch, Low Sweep, Stone Edge, Substitute, Focus Punch, Leech Seed, Seed Bomb....good god, I can't think of many things willing to switch in on the first five moves, much less the rest. Its movepool looks limited but it's all it really needs...well aside from a speed boosting move, but I can bet we'll all agree how something like Agility Breloom can shake up the metagame.
Next, Breloom's typing. This is kinda where it gets awkward. Breloom's defensive typing is fantastic, but frankly doesn't have the bulk to make full use of it, even if running a defensively inclined set. Its power is massive, but its STAB combo is resisted by so many pokemon. However, it is this very same power that makes even resists think twice about switching in. This is not factoring in Stone Edge, which easily blows unsuspecting Breloom checks clean off the field. Even the resisted moves themselves can really cripple Breloom's switch-ins: the likes of Gengar or Lati@s really don't want to take more than 3 hits of Technician Bullet Seed, while most switch-ins can lose their speed advantage over Breloom due to Low Sweep and become susceptible to Spore or a coverage move. On the other hand, some of these checks can succumb to SubPunch Breloom, which brings me to...
...Breloom's abilities. Now we know the true power of Technician by now: it allows Breloom to hit so phenomenally hard it isn't even funny, and grants it revenge-killing prowess rivaling Scizor, with more resistant opponents but also more vulnerable targets. And then there's Poison Heal, which grant Breloom double the Leftovers recovery and status immunity, making him a safe switch-in to the Scalds of Bulky Waters. This is also where his precise movepool kicks in: Leech Seed and Focus Punch combo extremely well with Substitute which is healed quickly by Poison Heal and allows him to pull off a mean SubSeed set, SubPunch set, or both *shudders*. The fact that most Grass types (even Venusaur) are wrecked by a STAB Focus Punch off base 130 Attack only helps Breloom's cause.
The cementing factor of this is that Breloom hardly has access to most of these great qualities when it was first introduced in Gen III, but was already established as an incredible force with Spore and Focus Punch alone (and that's without physical Grass STAB at the time!). So no, Breloom was never shit, it just got better and better with each generation and became increasingly harder to stop. Really, it's only flaws are its base stats aside from Attack and 4 moveslot syndrome, and that is pretty A-Rank to me. Breloom's not even as straightforward as he looks, and that makes him that much harder to stop.