I purchased an older unit that required a full renovation. It was about 13.5k per net square foot, and came out to be about 14.6k per net square foot after factoring my renovation cost. At the time Newer buildings of comparable size in the neighbourhood would have been in the low to high 20k per square foot.
Additionally, it might not be exactly what you want and you might be reluctant to renovate something so new. Since I was able to totally gut the place I was able really design it to how I was going to live in it as opposed to make due with the more generally appealing layout of a new build.
The downside is probably as nice as you do up your own apartment, you don't really have control over the common areas and the apartment will be a mix of different tenants. There are some neighbours who burn incense which can put off some people, the elevators are a bit slow, etc.
Additionally, lots of people will have the same idea, so that has meant that in my stay here, there have been a lot of other neighbouring units being renovated. This should be okay for most people as renovation is usually during the work day, but for me I have alternative work hours and am often at home when they are renovating. Additionally if you have a family or kids staying at home, they might be subject to the noise.
Also, renovation works can occur during Saturday, so if you're out late on Friday night planning to sleep in on Saturday morning, neighbouring renovations could mess with those plans.
So in a nutshell, I think overall it is much cheaper and you get something more tailored to what you want. That being said, facilities, and common areas might not be to the same standard and there's not much you can do about that.