I have volunteered twice with Frontier, once on the adventure project in Madagascar as my first big trip after school and once in Tanzania on the dive project. In both trips I undoubtedly learned a lot and made many good friends with those on camp but I couldn't help get the feeling that someone was 'off'. The more time I spent with the organisation, the more I learned that generally the staff in field are great but London seemed not only unhelpful but also inept.
In both places but particularly in Tanzania the accommodation was basic, makuti (woven palm leaf) structures with open doors, termites and spiders as roommates. There were two temperamental solar panels on camp and the kitchen was made of makuti. Considering that the project has been situated there for at least 5 years, numerous people have requested improvements -- and given that volunteers pay for these experiences -- I am still shocked that the accommodation has not improved.
Undoubtedly I learned a lot from my experiences, as someone now involved with other international developments and NGOs most of what I took away was what I kind of organisation I never wanted to be involved in. My review may seem biased but a quick google search will tell you a lot.
I did enjoy my time on projects, but 100% this was due to the hard work and attentiveness of in-country staff, as an organisation I would now not touch them with a barge poll. I would not recommend them to any individual as there are hundreds of more useful ways to benefit communities in other parts of the world. If i did have to give advice to anyone looking to volunteer in the future it would be to consider WHY you are volunteering. If it is to travel and meet cool people. 100% go with Frontier, but if you want to benefit a community in a supported environment do not. Have a look at the Sustainable Development Goals set out by the UN and judge for yourself whether this organisation really has something useful to offer to the community within which it operates.