- more work to set it up
- more paper work
- unnecessary in some cases
Pros:
- cash flow for you as a founder is more stable / taxes can be calculated and predicted more easily (only relevant if you expect a good year and then a bad year)
Neutral:
- Your personal liability is supposedly limited. This is only partially true. There's still the liability of the owner of the company (Geschäftsführerhaftung) and if you do illegal stuff, they can still hold you liable.
- Also for a GmbH, it is more important to manage risks by getting the required insurances and NOT rely on "limited liability". For apps, this would be "IT-Betriebshaftpflicht", possibly something about data protection with regards to GDPR violations. Maybe add insurance that covers personal liability so that you don't lose your personal cash if something goes wrong. That's about it. Optionally add domain-specific insurance if your domain is complex.
This is, btw. a typical German question. Mulling about the type of business is secondary. Get paying customers first, you can still incorporate a GmbH later. On the other hand: If nobody pays you money, you don't need a GmbH or be a sole proprietor.