It is never easy to make all the website ADA complaint. Also, most of the websites are not ADA compliant. I share my personal opinion on this question. Some websites ADA complaint is the must. Sites like banking sites, maps, ATM's, social media, hospital sites, and some more sites need to be ADA compliant. These type of should be universally accessible.
Before developing a website know about the ADA and WCAG guidelines. These guidelines are available on the official websites. After developing a website it is essential to check the website's accessibility. There are some tools available to test the websites accessibility compliance. But using these tools we can't get the accurate results. Manual+Automated testing may provide the closer results.