Do not do an Einzelunternehmen to start. Do a UG. Not worth exposing yourself to the unforgiving and extremely aggressive Finanzamt when they decide they want to collect money you didn't expect to owe.
My tax adviser told me I should go for a Einzelunternehmen (especially use the Kleingewerbe during the first year) because of the reduced costs (registration costs 25EUR and thats it) and not having the overhead. I’ve heard that registering a UG costs about ~500EUR + yearly costs of around 1500 EUR.
Given that this is during a beginning to try things out - without having too much money laying around to play with it.
Would definitely like to know about risks before getting myself into it.
If that's what you want to do, then a Einzelunternehmen (without Handelsregistereintrag) should be fine and less beaurocratic overhead. Especially for bootstrapping or side-projects it's always useful to have a Gewerbe registered.
As long as you follow the best practices (specifying 0€ expected income in the first year, etc.) and don't try to do any financial trickery, Finanzamt also shouldn't be a problem. However once you start to turn a decent profit (100+k/year) and/or start to hire people, you will be required to do more bookkeeping anyway, at which point it makes sense to found a GmbH (or even two layered ones).