Personally, since I've had "personalized" vehicles over the years, my threshold for where it crosses from expression of individuality into danger may differ from others who have commented.
I think a tastefully lifted truck with decent looking aftermarket wheels that work with the stock brakes and perhaps some aggressive looking tires can look good. Where things go wrong is where we get into the extremes:
- Aftermarket HID retrofit into stock reflectors, blasting blinding light all over the place
- Light bars, used in town
- Huge lifts that put the bumper well above the crush zones of other vehicles
- Aftermarket "roll coal" tunes that belch black clouds, often accompanied with the aforementioned lift above, which results in this taking place at window-level
- HUGE wheel/tire combos that showcase the wholly inadequate baby brakes that are expected to stop them
People drive like morons in everything, so I'm not going to lay that exclusively at the feet of "Bro's". Some of them drive quite reasonably, even if they are guilty of the above unsafe mods. Others do the above AND drive like morons however, which makes them more dangerous that Betty doing that in her Caravan.
The worst accident I've ever been in proximity of was a result of the opposite trend: the "slammed" crowd. A couple of guys in a Civic with chopped springs took an on-ramp on the 401 like they were driving in Formula 1. Merged at an extremely rapid pace, "carved" their way over to the hammer lane, during the process of which, a tie-rod end let go, sending the car into a spin where it careened through the median, rolled, ended up on its roof in oncoming traffic, and while still going at quite a clip, ground off the roof, and then proceeded to smear its occupants all over the 401 for quite some distance.