So here is what happens. Dealing with clients is tough. Sometimes you 100% know that what you’re proposing is the right thing but still they will reject it. A few years of working with clients has taught me that…it’s always good to make your client happy at your own expense. As long as they are happy…you’re good. But also, before going forward with your proposal…you should state to them clearly the advantages and disadvantages (if any) of any CMS you choose. Clients want to feel that they’re in control…and sometimes it’s better for you to let them use what they want…when they get stuck…they will come back to you and appreciate you even more.
So that’s one…Learning to understand the balance between what you want as a developer and what the client want.
Only recently I have got into WordPress Theme Development…You’re very right it’s so much better and easier to come up with your own themes from scratch…especially when you have already mastered that. But here comes the dilemma though…when you make those themes…do you make every area customizable? Like Navigation, Gallery, or sometimes even Layout? How do you handle customization with your own themes. And here by customization I mean…so that when you give your website to your client they should be able to edit those things…because we know they want to able to edit everything!
Last but not least, I think you need to counter their arguments with the exact opposite of what they say…not necessarily saying about other things or advantages they might not care about. For example, when client 1 say WP has more features - You can counter that by saying, “right…WP has more features but with more features comes more complexity…and that sometimes make WP hard to work with. But if you’re fine with learning WP…I can go forward with that.” That way they know you also care about their choice and at the same time you have explained to them about WP features and learning curve.
And when client 2 says WP is better for SEO…You can tell them…“Ok, I understand…So is SEO the most important thing for your site? If yes, then you might want to learn that there are other platforms that are actually better for SEO and WP that’s not its selling point.” You can mention Ghost, or just building static sites. If they’re still adamant or they have not understood you…don’t force it…Just give them what they want. Of course, you may get disappointed…but hey they pay you…and you need to make them happy even when their choice is not right.