I don't think that there is a difference in terms of liability: Both UG and GmbH are limited liability companies. However, my understanding is that an UG is obligated to retain a quarter of its annual profits as reserves until it has reached the capital requirements of a GmbH, at which point it can be converted.
IMHO, UG are a fudge. They recognised that the barriers to GmbH formation are high but at the same time did not want to drop them so they created something new that allows people to start a company before being able to afford a GmbH.
This contrasts to, for instance, the UK where limited companies (Ltd) are extremely easy and cheap to create and maintain (£1 capital and about £15 cost) so that there is no need for multiple types.
