BST is barely good anymore in old content (most jobs already outdo BST in old content). BST is terrible in anything adoulin related.
Abyssea, SMN and PUP both greatly outdo BST (and for a few NMs like cactuar and sandworm, SMN is the only one able to deal with them easily). Astral Conduit is superior to Unleash (except for cleaving) in most cases (BST pets deal 2-3k damage at most while SMNs deal 10k+ damage per BP).
Also even in cases where SMN needs to AoE everything down, Astral Flow still rocks. In reives for example, one aerial blast takes everything out (8k damage to everything). (Plus the AoE is 2x bigger than anything BST has too.)
(Yes, Unleash does last twice as long though but still, the damage output is 3-4x less than SMN with astral conduit.)
Dynamis, DNC/THF, THF/DNC, and even PUP/THF (they can farm WS proc enemies easily) all outdo BST (which is stuck with TH1 at most; if they go /THF they can't really proc).
VW - BST has to resummon and use up a jug each time a new VW starts (while a minor annoyance, still an annoyance). Going SMN is easier (plus, you can help proc staff and/or club as well as waiting for when a specific avatar's weakness is up).
Also, a BST with Aymur literally gets out DPSed by a SMN with Nirvana.
BST/WAR or BST/NIN vs SMN/SAM = SMN/SAM literally beats BST at meleeing.
Ruinator is one of the best one-hand weapon skills in the game (especially with weaponskill gorget and belt) but BST still lacks native dual wield (and you won't cap -delay without dual wield or without a DNC with 10% haste samba) and using /NIN kind of sucks (plus it doesn't help Aymur).
Not only that, BST is still stuck with the horrible "Beast Affinity" problem - AKA not being able to use any pets. (Yes they have been releasing HQ pets over time but instead of having to wait 2-3 months to use <x> or <y> pet that BST could use 10 levels ago, why not just uncap all pets and change BST affinity to something else?)
If they continue with increasing item level, BST will have yet another period where they are stuck with 3-4 pets (while the rest of the pets are still stuck at some low item level, though with BST affinity and monster gloves +2, BST does have a +15 level cushion they can back on but still, it is really silly).