Hi Glyn,
Thanks for such a thoughtful post, it looks like you are really invested in finding the right solution. I’ll address your questions here:
Stability and Reliability
I can’t personally speak to Knack’s reliability a few years ago but what I do know is that we have a large customer base who have been using Knack for 5+ years, and I think that is a positive sign. Since I’ve started a few months ago, Knack has taken a path that, in our current economy, is a bit radical and the opposite of what I’ve seen other companies doing (investing in Sales) by dramatically scaling our Engineering, QA, Support, and Design teams. We have more than doubled just our Engineering team alone.
We have projects in flight now purely focused on performance improvements and reliability gains because we want to continuously get better.
Sexy GUI
Knack has so much power and functionality, especially for data-driven applications, and it was created with the same flexibility for the front end, allowing users to add their own CSS and JavaScript. This forum offers a ton of great examples if you want something specific. There are extremely skilled and knowledgeable Knack Partners that build out custom solutions, and I’m sure a few would be happy to show you their work. All that said - we have plans to update the GUI for users who don’t want to use custom code too. 
External APIs
You can connect to external APIs using Knack, and although there isn’t a built-in solution yet like Bubble’s, you can still achieve this without much effort. User-friendly platforms such as Make.com and Zapier offer powerful integrations with Knack and can help you connect to external APIs with minimal technical expertise.
Disappearing Tables
We’re so sorry you experienced this. This would be a great reason to open a ticket with Knack Support. Without more information, it can be difficult to diagnose technical issues in the community forum.
Mobile-friendly
Knack is mobile-friendly and will adjust to your screen size, but there could be certain layouts or elements that may not display as well as they could on mobile devices. When building the Live App, there is a cell phone icon towards the top of the screen so you can preview the app in mobile view and make adjustments. It’s also possible that by adjusting your app with a bit of CSS and making sure all components are optimized for mobile devices can mitigate these issues. You can also give a shout out to the Knack community for advice.
Copying Records
It’s not out-of-the-box yet, but now you can create a custom workflow using our guide here or by using a bit of JavaScript and the Knack API to duplicate records.
Big Data
I’m not sure how large your datasets are, but you can import data (or update existing data) in bulk from .csv files up to 250mb. Records can be added via the API as well. I’m interested in chatting with you about the big data more if you’re open to that.
Roadmap
You wrote this message at a great time!
We just today sent out our April newsletter, hopefully it’s in your email.
Also, keep an eye out for an announcement about an upcoming webinar where I’ll talk about our roadmap. 
We want to drive growth with our roadmap, of course, but it is our intention that your clients will love the changes - and as the person building their solutions, we want you to as well.
We want to solve real problems our customers have, and we can’t do that without your feedback and input. We’re a small but growing team still, so how we gather the feedback could be reading the forums daily, sending out surveys (there is one in the announcements channel now that could be interesting to you since you asked about connecting to other data sources), getting on calls with customers, etc.
We look forward developing a collaborative relationship with you (and of course all customers 
)- reach out via the forums, or send a note to product@knack.com.
Have a great weekend 
Cheers,
Kara