Find and Compare Aero Bars for your Cycling Rides & Competitions
The disadvantages are mainly in the area of aerodynamics because a one piece aero bar can be designed from scratch with wind performance in mind. This might only concern you if you are in the top levels of your sport though.
These give you the use of an aerobar without having to start changing the bike handlebars, but nevertheless there are quite few styles and shapes to choose from. The extensions are very important and even if you are just starting out with aerobars, its worth having some ability to change the length of the extension.
Also have a good think about the shape of the end of the bar. Are you going to be pulling on the bar to accelerate rapidly or are you likely to stay down in the aero position for a long time. If possible try someone else's bike that has an aerobar, just to get a feel for what is going to suit you best.
There's a big difference in demands placed on a clip on tri bar compared to one that you might use for TT races.
If the aero bars are going to be used in anger frequently, then also check whether the clamp is reinforced, as it will take up a lot of force. You'll find that there are stem clamps available from some manufacturers that allow secure bolting around the existing stem. Various extensions can then be fitted onto the clamp.
If you have an existing carbon handlebar and are intending to clamp on an aerobar, then be very careful. Some carbon bars are not made to take the clamping forces you will apply and others are not made to allow sufficient grip to stop the bar rotating. The last thing you want is a moving bar or a fracture to happen mid ride.