When it comes to rooftop tents (RTTs), selecting the right crossbars is crucial for ensuring safety and durability. Two key points to keep in mind
Heavy-Duty Crossbars: Opt for crossbars with a minimum width of 8cm, especially if they're made of aluminum. This ensures they can support the weight and stress of a rooftop tent.
Avoid FlushBars: For RTT support, it's important that the bars hang over the bar seat. FlushBars, which sit flush with the vehicle's roof, are not suitable for this purpose.
the one shown above is a flush bar as the bar is flush with the leg, however, this is not optimal for rooftop tents compare to through bar as shown below. 
Through bar is more optimal for a rooftop tent, the weight is more evenly distributed across the leg as the bar is through the leg and is wider allow for more balanced support and allow more space for attachment.