For me it comes down to bugs. Less flashy means less likely to glitch on me, makes me feel safer. Whether or not it's true, I'm not sure.

I think there is a sweet spot.

Too "modern" and I assume a site is using excessive amounts of tracking and playing other tricks with A/B testing different marketing tactics for services to try to milk their customers for all their worth.

Too crappy and I start to question if the site is actually secure and if they are putting enough money into IT... or if it's even the real site vs some kind of scam.

Both can be problems that erode trust, but they are different problems.

It is possible to have both great content and great design, but it's rare. Think of the Met museum website vs a tech startup. Both will have modern, fancy UIs, but the museum also has real information where the marketing site will only have design elements and simple hype statements.

That said, you probably want to judge your bank's trustworthiness by it's rating with the Federal government, which is public info. Most banks (all but the biggest) have to outsource their online banking software and it's never as slick as the big guys'.

I know that in general, the flashier a thing is -- that is, the more obvious effort that has been put into a thing's aesthetics -- the less likely that thing is to be great.

My hypothesis is it's because time and money investment was put into aesthetics at the expense of making the product or service better.

How long have you been using your bank and its website and how satisfied are you with their service such that you are overlaying your lived experience onto your assessment of the trustworthiness that the website projects?

We're trained to assume that the prettier things are implemented by entities that care more for the pretty than the function the thing was supposed to preform.

Uglier, less decorated things are expected to function better, with less effort "wasted" on goals unrelated to the problem being solved.

I trust sites that aren't heavy with javascript (less ad tech and so better privacy). In my experience, banks sites are often optimized to run on low end user devices and so make good use of old fashion HTTP requests for managing interactivity.

People are cautious around change, it's comforting if something stays the same. If the bank's website has been the same for 10 years, it must have been working well enough to not need replacing. A new flashy website is unknown and untested.

"old" vibes, old -> trustworthy, primitive instinct

They have priorities other than following the latest design fad?

That, and they haven't yet fallen into the trap of giving their marketing department too much say.

The bank's trustworthiness doesn't come from their 98 Lighthouse score. It comes from the fact that they have the word "bank" in their name, and people trust banks.

I think "crappy" implies an older, maybe even pre-web 2.0 design. This suggests the site has already been around for a long time, specifically before all the surveillance capitalism and general enshittification trends started. If they didn't update their design since then, that would give some hope that the site still runs more or less unchanged since back then.

This is how I'd explain the impression of trustworthiness. But of course this is only an impression and doesn't tell you anything about how the site actually operates.