I found 'Treasure of Matecumbe' just about entertaining, helped by the impressive main cast.
Peter Ustinov, after a poor showing in 1975's 'One of Our Dinosaurs Is Missing', comes back strong with a top performance as Snodgrass. Joan Hackett is also enjoyable as Paxton, especially across the first act. Robert Foxworth fits in too, as he plays Uncle Jim. The two kid actors are Johnny Doran (Davie) and Billy Atmore (Thad), they aren't amazing but are at least likeable and are passable in what they give.
The plot is where it becomes less interesting. The adventure elements are decent, but it does plod along at a number of moments. It doesn't really feel like a treasure hunt until the last knockings, which is disappointing - especially given the onscreen talent. Parts of it also haven't aged gracefully, while the vibe of the film changes from serious to fun too often; e.g. you have a KKK scene, but also a runaway bride bit.
With all that noted, it is still a film I can say is good enough and one of Disney's better live-action releases from the 1970s.