Usually they have a number of limitations: on bandwidth, on traffic sent and received. The connection is not reliable and can be interrupted with no reason. There is no support at all, or poor support as a rule. They are usable, of course, if you need VPN connection for a simple and short time task, e.g. to unblock a site, to send a message or to download a small file.
However a free VPN server can hardly be used for day-to-day activities requiring permanent stable connection with no bandwidth limits.