When I was looking for adjustable tie-bars for Shed to try and sort out her uneven rear ride height, I settled on Cookbot Automotive's stainless steel adjustable tie-bars, with Forge's tie-bars coming a close second, mainly on price since they're both very well-made products.
I wanted to fit two pairs so I could get the rear geometry perfect; Shed had an at-the-time undiagnosed problem with the rear-left suspension being 20mm lower than the rest of the wheels, and was on the limits of her rear suspension's adjustment in a couple of measurements, so fitting these arms would give me the best chance of getting everything back perfectly into spec.
I didn't feel happy about using the cheaper-made tie bars I'd seen for sale on eBay, primarily because I had no way to check their quality control; I would have no knowledge of the quality/purity of the metals being used, and I wouldn't know whether the person doing the welding knew how to make a good, clean weld that would remain strong after ten years' abuse by salty roads.
During my research, I saw a few photos of snapped tie-bars; it struck me that the common failure point seems to be at the thinnest section, which would often be where the threaded adjustment bar was taking the entire load.
Considering the lateral forces which can be exerted on a tie-bar - particularly the lever force on the lower arm when it's on the outside of a hard corner - I can see why they break.
As an example: considering the threaded bar has a cross-section of - for example - 12mm, and say you're cornering at 0.5G with a TT's rear end being, what, 600Kg?
My rough-probably-wrong maths gives me a lateral load figure of 265Kg per square cm.
That doesn't take into account the vertical force of the car's weight, or any allowance for the shock loads of hitting a bump or kerb sideways.
If, say, you hit a kerb, that could put a significant shock load through the bar; for a shock of 5g, that would take the load up to ~2,500Kg (2.5 tonnes) per cm squared
The Audi OEM arms might look like they're not very substantial, but I have Shed's used set here and they're very thick metal. At no point does their cross-section have any obvious weak points - they're a box section all the way through, so loads are evenly distributed across their width.
Being mild steel, they will also have a degree of flexibility and resistance to work-hardening which a cheaper grade of stainless steel wouldn't have.
I'm very happy with my Cookbot arms. I fitted little neoprene gaitors over the adjuster threads per Paul Cooke's recommendation, and doused them in ACF50, so hopefully they will out-last Shed.
You can find out more about Cookbot's adjustable tie-bars here: