Ok, so first of all, the UG was only introduced in 2008 - which even for some of us middle-aged people is more "it's relatively new" than "GmbH has existed decades before I was born".

Second, not that I claim to be any representative, but this "UG is weird, unknown, not serious"... I have personally not heard that for about 10 years, so yeah, absolutely true when it was new.

Third, it's still relatively rare I'd say. You stumble over one every couple of months or years, in my experience mostly online shops without their own manufacturing (but maybe that is where I am looking) - if you just look at what tradespeople have on their vans: Nope, never. Always GmbH or GbR/single person.

But I have no insight into procurement departments.

You’ll find that many people don’t have 25k€ to invest in initial capital of a business, but that are perfectly capable of running a business regardless. 5k€ is fair for plenty of endeavours, and more should be required for riskier businesses.

People seeing a GmbH as more serious is just a cultural aspect, I’d say.