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 domain does not belong to you and the paid one does.

If we talked about free hostings then we would have to be careful, since you do not know who can have access to it and you can get a surprise. But with the domains I would not worry too much.

I have used the .tk domains as temporary domains or for tests and I have never had problems.

Generally you have to assume that nothing is free. If you look at the business conditions, you realize that you can register all domains that are free for a maximum of one year, for reservations beyond there is a price list. The prices are quite competetívo (for now), but nevertheless the page is little transparent in the aspect of the general conditions of business.

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