Client: it's very important that we go live with the app's sign up process.

Me: ...but the mobile apps aren't complete yet.

Client: Yes, but marketing says we need to start getting downloads and sign-ups so we get better ratings than our competitors.

Me: ...but the user won't be able to do anything.

Client: We'll add the rest later when we have users!

I'm not a Wordpress pro, but i've done some work on it and... well, the HTML + PHP mix is the only evidence you need to convince someone, there is no god, only satan, but other then that, it's hard to find anything else that your clients will accept & can use. And yes it's the most important thing - at the end of day, they are the one giving you money.
Besides:
- the Wordpress dev community is pretty cool.
- everything has been invented, so the development is pretty easy.
- if you have something more fancy to do, there is always a Wordpress API.

PS - fuck HTML + PHP mix.