They don't function in that they cannot use the primary sanction a union has against an employer.
Which is ultimately to go on strike - the workplaces just aren't heavily unionised enough
So you think a union is only effective if they can force a business to practically shut down for a day or two? If that's the case why do unions (which could force a business to practically shut down) opt to take action like overtime ban, ban on rest day working etc.?