No prospect of paid roles for volunteers and no opportunities for progression. Oxfam has the luxury of being able to recruit volunteers from an inexhaustible pool of young unemployed people, graduates and others who are down on their luck and who will often put in long hours over extensive periods of time (and so will have all the skills and experience required for the job). Oxfam appears disinterested in creating paid roles as a result. Volunteers are disposable, with few staying in the long-run as the roles are only really helpful as a stop-gap. Meanwhile, managers are paid disproportionately large sums. This does not seem fair.
Can also feel demoralising if you are volunteering and are not fully appreciated, or chastised unfairly. After volunteering for around a month and moving on to a new job (which I informed the manager about, explicitly explaining I would be unable to return for the forseeable future as my hours were uncertain, but I might be able to work something out at a later date), I received an angry and patronising phone call a week later, pushily demanding to know why I hadn't informed them if I would be returning by then and reminding me that I had a "commitment to the team".
This bizarre incident, as well as some of the impatient/rude customer reactions I received when I was giving up my own time for no reward, made me wary of going into similar voluntary positions in the future.
Staff are not really made aware of global issues Oxfam is supposed to be tackling, and how much of the funds raised go towards the cause itself is dubious. Certain business practices could be made more ethical e.g. lack of recycling facilities, products can be priced unfairly (using online prices and adding their P&P fees to calculate the Oxfam item price), aforementioned lack of benefits for volunteers.
