For the Feats: Ok, it's a bit better if they can only stack 2 feats, but something like Improved Toughness can be used multiple times and has no prerequisites. For a mere 45gp per feat I can get as many as I want to buy, making me Immune to damage through having so much of it. Except for maybe a few Ubercharger builds or the D2 Build that gets NI damage I can ignore it.
No Constitution score. (Doesn't apply except against harmful effects that work against CON)
Darkvision out to 60 feet.
Immunity to all mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects).
Immunity to poison, sleep effects, paralysis, stunning, disease, death effects, and necromancy effects.
Cannot heal damage on their own, but often can be repaired by exposing them to a certain kind of effect (see the creature’s description for details) or through the use of the Craft Construct feat. A construct with the fast healing special quality still benefits from that quality. (Can heal damage on their own but the other ways work too)
Not subject to critical hits, nonlethal damage, ability damage, ability drain, fatigue, exhaustion, or energy drain.
Immunity to any effect that requires a Fortitude save (unless the effect also works on objects, or is harmless).
Not at risk of death from massive damage. Immediately destroyed when reduced to 0 hit points or less. (Not destroyed at 0, but immune to massive damage)
Since it was never alive, a construct cannot be raised or resurrected. (Ignored)
Because its body is a mass of unliving matter, a construct is hard to destroy. It gains bonus hit points based on size, as shown on the table.
Proficient with its natural weapons only, unless generally humanoid in form, in which case proficient with any weapon mentioned in its entry.
Proficient with no armor.
Constructs do not eat, sleep, or breathe. (Except when they want too)\
Only counts as a Construct when beneficial, Harmful spells must affect both (original type) and Construct to effect the Brick Builder
Those are the traits I'd adjust. Others may have a different opinion for various reasons.
But that's the point, unless you stat out what they get there will be arguments from Players and DMs about what exactly is a beneficial trait.