Hi,
I don't have a Juwel auto-feeder but I do have a Fishmate auto-feeder. Irrespective of brand/model you opt for, I can't emphasise enough how important it is to test it out for at least 24hrs beforehand and ideally for several days. That will give you an idea in practice (as opposed to what the box says) of which foods go through and don't get stuck. My auto-feeder has, over time, and not because of depleted battery power (I always put new batteries in it just before use) or malfunctioning in any other way, developed its own idiosyncrasies eg it's necessary to adjust it in such a way that it's on the verge of depositing the first feed rather than at the intuitive position otherwise it actually misses out the first feed at the appointed time and waits until the second appointed time to do so. Another issue is that, over time, more moisture/condensation seems to be in the air and affecting the food quality in the auto-feeder by making it soggier - I now need to leave the top cover of the auto-feeder open in order to minimise this whereas this wasn't a problem in the past. Best to discover these issues before rather than after the event.
I don't know anything about the Juwel brand, but the Fishmate auto-feeder takes flake and pellet food (generally, the bigger, the better, as it's less likely to get stuck and clog up the deposit slot altogether, which has happened with flake food quite a few times in the past) and should take algae wafers (but I haven't tried that); it definitely wouldn't take frozen food as that would defrost before getting to the point of being deposited through.
Hope this helps.