I take a lot of pride in my DIY skills and a few years back, I decided to go it alone during installation when I was replacing my old a/c unit with a newer model. The short and long of it is that I had a very horrible experience and had to finally get in a pro to fix my mess.
When I finally called up a professional company in Crawley, they were able to fix the leakage in the ducts and address the airflow problems I was facing. I immediately saw better performance from my a/c.
I have also come to learn that some manufacturers might void your warranty if you don’t use professionals during the installation process. This is why you shouldn’t go it alone during a/c installation.