I'm only reading from other brewers who are brewing with the Speidel Braumeisterthat they it is very difficult, maybe even impossible, to brew beers with high alcohol levels. Is that true?
It depends on what you're looking to do.
I've seen the 20L system handle ~1.070-ish beers without trouble. That's a reasonably big beer, though many people ignore the included instructions and start doing things like sparging to get there.
If you want to go much above that, however, I get the impression that you'll need to shrink your batch size or use an extract of some sort to supplement after the mash.
I have some friends that have the 50 liter version (about 15 gallons) and I know they've brewed a couple of RIS and other strong beers, so it's definitely not impossible, however I don't think it's as easy as some other setups.
They had to perform two separate mashes due to the large amount of grain. Each mash had 10.5 kg (about 23lb) of grain in order to hit 1.094 OG. Efficiency was around 70%.