Indeed, any grid trading system has its risks and you must be aware of them when trading it.

I think there is no danger that the evil SNB suddenly removes the price floor at a weekend. The Swiss won't remove it without good reasons and lengthy discussions in parliament, and certainly not at a weekend. So you had plenty time to get out in that unlikely situation. But there are two real risks involved in such a system:

The first one is that the CHF moves in only one direction or not at all for a long time, leaving you with open positions that cost you rollover every day if they are sell positions. The other danger is that the EUR/CHF rate has a floor, but no ceiling. If the swiss economy collapses, the rate will climb, and it's a bad thing to have open sell positions at 1.25 when the rate is at 1.50. You can avoid those risks when you restrict trading to long positions, but this requires more capital and reduces the profit. Although I think the Z5 risks are smaller than the risks of most other systems, they are certainly not zero.