European countries offer a variety of different unionisation models, so provide reasonable evidence on the usual consequences of unionisation. Dutch military personnel are represented by a union, for instance.
The European Association of Military Organisations appears sanguine about the idea, although many European military unions don't have the right to strike (emphasis added):
The President concluded by emphasizing that EUROMIL strongly believes
that military trade unionism does not harm military efficiency. On the
contrary, being represented in trade unions and professional
associations can enhance the [morale] and loyalty of the troops.
Less information is available about the consequences of unionisation on working hours; in general, it seems that unions appear to understand and accept the reasons for long working hours under certain circumstances.