Is it safe to use free domain names?

A domain, regardless of whether it is free or paid, is nothing more than a name that points to an IP address. In general, the only difference between a free domain and a paid one is that the free one does not belong to you and the paid one does.

If we are talking about free hosting, then we should be careful, since you do not know who can have access to it and you can be surprised. But with domains I wouldn't worry too much.

I have used .tk domains as temporary or test domains and have never had any problems.

Generally you have to assume that nothing is free. If you looked at the business conditions, you realize that you can register all the domains that are free for a maximum of one year, for reservations beyond that there is a price list. The prices are quite competitive (for now), but nevertheless the page is not very transparent in the aspect of the general conditions of business.

So if you want a domain for a private project you don't risk much, but if you want to set up a business it's worth thinking about it, because for all free domains it's worth it: freenom remains the owner. That means there is no guarantee on the price in the future.