I look at things like this with a bit of skepticism.
Hi Henrik,
Forgive me resorting to English here- I'm the (Australian) developer of tripwaffle and thought I'd take a shot at addressing some your concerns.
For way of background, I have ~30 years of experience in app development, including being the technical founder of a NASDAQ listed web app (google "smartcompany retailmenot Guy") with over 100 million visitors a year.
tripwaffle for me is a labour of love, as is most the development work I do now days. I also travel a lot, hence the passion for this particular project.
I can confidently say, my current agile approach to development, most certainly beats the alternative approach I've witnessed working at large enterprises (nothing gets done), plus creates a critical connection to early users.
In terms of specific security concerns, I'd say the two main risks here are:
1. malware in app
The official app stores have an ongoing cat-and-mouse battle to keep malware out of apps (with entire teams of researchers devoted to just this). They've gotten better at it over the years and now any apps published undergo pretty thorough security checks before being published. Not entirely bulletproof though, so you probably should rely upon reputation to an extent.
2. data / app security / privacy
What app publishers do with users' data is somewhat out of the hands of Apple and Google, so this once again comes down to reputation to a large extent. It's not exactly a great signal either though; as many large organizations have suffered data breaches, as you know. Much of that risk surface is incompetence or too many people with access they shouldn't have.
Hopefully I can help on the reputation front by providing a little more info about myself. Probably the most relevant part of my bio here is having been a board member of rome2rio (google "rome2rio.com 2019-11-01-Rome2rio-aquisition-announcement.pdf").
Anyway, hope you take the time to assess tripwaffle on its merits, and please shout if I can address any other specific concerns.
Cheers,
Guy
P.S. I dont have the privileges to post direct links, apologies