No hard work in the army for draftees. Nor on most Zivi jobs. The most you can argue it's a waste of time. But most people leaving school don't really know what to do anyways (hence why the FSJ/FÖJ has become so popular ever since fewer people get called up.)
It's not comparable in any way or form to slavery, at least as the west knew it - and I'm including serfdom here.
It's also rather insulting not just to those not many generations away from having been slaves, but also to draft veterans. Or former Zivis, might I add.