After thinking entirely TOO hard about what class I wanted to go to with my main character, I formed my own opinions on the subclasses. There's some weird balance issues with some of them.
For example, ALL of the Barbarian's classes are kinda ****. Berserker has some amazing buffs with Frenzy, but they lose out on the final "upgrades" of Frenzy that can extend time as long as you keep killing. Corpse Eating just adds another bit of healing in an already pretty damn full healing repertoire. Mage Slayer is just about useless.
Except for Evoker (or Transmuter for certain builds,) all the wizard classes kind of suck. The entire point of the Wizard class is the huge amount of flexibility in the spells. Strangely, I think Wizards need a buff in general. Priests, honestly, make a better Wizard. To a certain extent, so do Druids (especially from a DPS standpoint.)
Blackjackets kind of have one big build they can do, but, other than that, they kind of just exist.
Soulblade exists almost entirely as a multiclass. It can't tank, the health pool is too small, and it has no real melee attacks besides one...that uses focus instead of building it. Honestly, so does Beguilar. If Ascendant didn't build focus as fast as they do, they would be in that category as well. But, you can at least make Ascendant a single-class that hilariously destroys everything after getting Ascendancy in 1-2 attacks with Frost Seeker. Mind Blades becomes very, very OP at that point, and you can basically dominate the entire enemy force since it doesn't cost any focus during that time.
Trickster almost exists so you can avoid making a spellblade, but the main focus of rogues, something even the Swashbuckler has, gets nerfed quite heavily.
In theory, all the subclasses are suppose to be balanced to still make main class appealing. In practice, nearly all the main classes are inferior to the subclasses, with the exception being Wizard (unless you're making an Orge-build that really should be a Druid or an Evoker,) Barbarian (as you only get the Frenzy upgrades if you take the main class or the two **** sub-class choices,) Druid (it's actually balanced,) and maybe monk (Actually balanced...except for maybe Helwalker due to Might being additive.)
Chanters have no real reason not to choose a subclass, Ciphers have no reason not to, Fighters are better off as Devoted or Unbroken, rogues lose out on too much not choosing on, Paladins/Priests have no choice, and Rangers have one useful subclass or class in general that is restricted to a companion.
Despite that, they aren't really poor design (except in the situations where not choosing a subclass is actually one of the best options.) Everything else is OP as hell. Except the Rangers. The Ranger class got the short end of the stick.