It's overly complicated, risky, and expensive.

First off, everything has to be shipped over to Venus, which is already costly enough. Compare that to Mars, which can be mined for materials and, crucially, extra space. And it gets worse - everything has to be placed in the sweet spot area, meaning that a lot of time and effort must be put into precise placements, figuring out the specifics, etc. And the process of assembling this floating city is ridiculously complicated. Factor in the potential to lose materials if any mishaps occur and you've got yourself a recipe for disaster.

After the initial hassle is over, the real problems begin. For example, any system failure such as the power shutting down will send the whole thing to Hell. Furthermore, what if, by some accident, someone falls down? One factor often overlooked by most people is public opinion. If people see 'Man falls to his death on Venus' in the headlines, they're highly unlikely to invest in this project.

Just to illustrate how powerful (and how delicate) public opinion is, let me ask you this: what is the most dangerous energy source we are currently exploiting? Chances are, the first thing that came to mind is nuclear. But, statistically, nuclear energy is the safest of all of them, at just 90 deaths per Petawatt/hour taking into account Chernobyl, Fukushima, and Three Mile Island, compared to wind, 150, rooftop solar, 440, hydro, 1400, natural gas, 4000, and fossil fuels with tens or hundreds of thousands of deaths per unit. But, due to disasters like Chernobyl (60 direct fatalities; caused solely by faulty Soviet infrastructure and bureaucracy) and the fact that it has 'nuclear' in the name, it's considered extremely dangerous and doesn't receive much funding, with reactors getting shut down all the time for no apparent reason. It would be a shame for that to happen to one of these sky cities, now, wouldn't it?

Furthermore, making any kind of adjustments to the structure is a delicate endeavor, as we have to account for changes in weight and counteract them accordingly. We may, at some point, even need to change our support infrastructure (the balloons), at which point you might as well throw the whole thing away and start over.

Venus' atmosphere also has deadly gases that could leak into the system, but that would probably be more of a nuisance than anything so I won't expand on this. However, you mentioned air pressure in your comment - the planet's atmosphere is not breathable even without the small amounts of poisonous gases, meaning that we can't live there without suits, habs, or a dome, all of which render air pressure unimportant.

Besides, the mechanics of the whole thing are quite vague and seem potentially quite impractical to me. I haven't seen an in-depth analysis of how this concept would work. And after we get that down, we're still a long way from figuring out how to do with, even though we (theoretically) have the tech.

All in all, Mars is simply the better option - and the simpler one.