I've never worked on a CMS I couldn't perform effective SEO on or rank. I've taken crappy GoDaddy sites from nowhere to #1 for search terms. Of course it has some limitations, but in the end it's not THAT limiting. The fundamental elements I need access to are all available. Most SEO's prefer a certain CMS like WordPress just because it's familiar to them. That's the lazy route. Don't be afraid to learn new platforms. I actually enjoy learning new platforms. You should also be considering the reason they are on that platform in the first place. If it's ecommerce, there's tons of functionality to consider that may be entirely reliant on a platform. Imagine having to migrate a Magento site with 5000 products and crazy customizations to a WordPress + woocommerce combo for the sake of SEO. That could literally destroy a business. So be careful on your recommendations I think if you're taking on a client, and a new website is in the cards, sure you can recommend a platform you'd like to work on. But otherwise, if you're good at what you do, you should be able to make it work on whatever CMS they currently use. GMB doesn't care about CMS. Quality content doesn't care about CMS. Authoritative and relevant backlinks don't care about CMS. Hell, there's a lot of SEO you can do without ever touching a site.