The program (DSM) crashes all the time, sometime when by simple mistake you try (for instance) to remove a face or part of a solid or while moving parts. It simply start processing something infinitely (sometimes if lucky it stops with a nonsense error like "operation not possible" ..roughly so).
In case you create a file and either if you not save it or if you save the file but the file was empty, after hours of work you risk to get blocked and nothing can be found in the backup directory not even if you programmed the backup savings to be performed automatically each 10 minutes (or less).
It is a free software which you may consider good for the money, but judging the time spent with its bugs it rather make sense to buy a professional product.
As I am most certain nobody takes care about the crashing issue (nobody did something from version 3), I am at least expressing my dissatisfaction after almost 2 years of insistence with this mechanical toy.