Strut rods are fine, even the rod itself will flex slightly where it's bolted, but it's designed to do so. The main load on it is held in compression and tension to prevent the arm moving on the longitudinal axis of the car. Even a boxed control arm or a tubular is going to flex to one degree or another. It's the nature of structural steel, and why it has high failure points....it bends to absorb energy. The strut rod can flex with little to no change in alignment, but it won't compress or extend enough to allow the arm to move any measurable amount.

That's why if you look at factory double a-arm setups, they use large cast or forged aluminum and not steel.....because the AL alloys are more rigid. They need the rigidity to account for a lack of a strut rod and the arm wanting to flex around, and cast AL is common in mass production now due to ease and low cost of production, and high functionality/rigidity.

A pivoting strut rod is extremely strong. Keep in mind all the Trans Am cars, and all the guys who do vintage racing are required to run them. That's on 800+hp, mega wide tired, fender flared cars with massive brakes and suspension, running on racing slicks.

Each system has advantages and disadvantages, and if it helps I'll just list them for comparison sake.

With a strut rod the advantage is it gives the greatest control over longitudinal arm movement. Good luck stretching or compressing a steel rod! It effectively triangulates the control arm to the chassis, similar to an a-arm, but since you're compressing and extending you can have greater control over bushing compliance since the strut bushing only has to work in a single plane. It gives a much greater range of adjustability over other designs for caster, which may or may not be necessary. The use of different bushing materials or pivots(rod ends for instance) with limited deflection can allow for a HIGHLY precise suspension travel control with the least amount of unwanted movement. Multiple geometric planes allows you to(when properly designed) exercise VERY precise control over the travel arcs of the suspension components, you force the system to remain triangulated at all times in all directions.

The disadvantages are it's a more complex geometric setup than a basic double a-arm. By adding an additional arc in a separate plane, the geometry has to be worked out so that suspension travel doesn't cause the strut rod to force the control arm to travel in unwanted forward or rearward arcs, so strut placement is critical. It adds additional parts, bolts, rods, bushings etc than may require more maintenance. Depending on the compliance of bushing material used, you may have poor control over the stability of the system. It is more complicated to align, which if the alignment tech isn't savvy, can be a problem as it's easier to get wrong. The more rope you have, the more rope you have to hang yourself with, as the saying goes.

With a double a-arm, the single biggest advantages are cost and simplicity. With simplicity, the arm itself locates the system. No strut rod is needed for triangulation, so longitudinal movement is controlled by the arm and it's 2 mounting points. It's a bigger matter for production cars, where it's easiest to bolt in, so from a production standpoint it's a HUGE cost saver in assembly labor. Also, many double a-arm setups have alignment designed into the system, so the car is properly aligned when assembled new, another huge advantage in production as it eliminates the need to align the vehicle. It's also arguably an advantage in real world application, where as long as the parts are undamaged, the alignment tech can't screw it up, so it remains in proper alignment. It has much fewer parts and therefore requires less maintenance. It's easier to package in many cases, since you don't have to worry about where to place a strut rod as you may or may not have the proper space and ability to mount one correctly. They cost a lot less due to fewer, easier to manufacture parts. Again, primarily a large advantage in mass production.

The disadvantages of double a-arm, they can suffer from a lack of adjustability in alignment. Even adjustable double a-arm setups, have a much more limited range of adjustment, making it more difficult to account for variances in driving preference/style, or even minor damage. A slightly tweaked anything that could otherwise be adjusted out without problem, can suddenly require replacement of parts due to an inability to adjust something 1-2*, or even 0.5* more. Longitudinal stability is entirely dependent upon a-arm rigidity and bushing compliance in the arms. Steel arms, depending on design, may flex more(which is why AL is used pretty much exclusively in double a-arm setups today), and more compliant bushings allow for more movement at the arm mounting locations, both of which can allow the lower ball joint to move forward or rearward more. Smaller, more rigid bushings and more rigid arms are required to maintain stability. Because the bushing controls load as the a-arm moves up and down as well as front to back, under braking/turning the bushings can become more heavily loaded in specific parts of the bushing as the arm moves front or rear and ***** the bushings in their bores and then the arm moves up or down. This can lead to accelerated bushing wear since the bushing is forced to handle more load. Due to the simplistic geometric design(which has it's advantages), it offers the least amount of control over suspension movement, since it has the last amount of interaction of different geometric planes.

Usually the decision to use strut rods vs a-arms is a manufacturing decision. It's a big deal from a production standpoint, but once a car is made with one or the other, there's really not need to change it. If a car has a strut rod design, provided it's a good design, it will give excellent suspension control and there's no need to change it. If a car has a-arms, provided it's a good design, it will offer good suspension control, and since it's often impossible to switch TO a strut type design without suffering a handling loss(a chassis needs to be designed for it really), there's no point.

The kit your looking at is affordable, and does offer the advantage of simplicity. But, you need to consider several key things that could pose VERY large problems. Is your chassis 100% straight? Or even "straight enough?" Even a slightly tweaked chassis or mount somewhere, could mean that you wouldn't be able to align this setup correctly, or would have to use a compromised alignment. How much handling from a compromised alignment would you lose? Will the company stay in business? Do they use parts that are commonly available, so if you need maintenance in the future you can get bushings, ball joint etc? How concerned are you about handling? What alignments can you run with this setup, and how much compliance is there in the bushings and arms they use? If you're looking for a big time corner carver, then this setup may not be for you, since the strut rod design will give you better alignment control and with the right parts will outperform a double a-arm.

Just things to consider. There are a lot of companies that sell a lot of parts that look great and work fine. But they're in the business of selling parts. Have you ever met one of these vendors that will tell you that their setup is NOT the best? You never hear about the problems these systems have from the maker, not until someone else uses it and complains.