The Not-So-Good
Lack of story hooks. The locations of the booklets are provided without any reason for the players to go there. That is left to the GM to invent. The locations generally do not have much of a plot or situation unfolding. They tend to be a location, with a few curious NPCs, some of whom might hire you. Again, some work from the GM is required. The Reach of the Roach God also has no treasure (really), just a race of insects wanting to invade the surface.
Lack of gameable content in the TTI booklets. Each of the Thousand Islands booklets has a few pages of interesting items, like magical fruits or curiosities from the region. But many of them were not really gameable. For example, Tohey Leather from Mr-Kr-Gr, is a waterproof fabric made from woven palm fronds. Interesting, but ... so what? Maybe you could make them into setting-specific and interesting trade goods for the party. A missed opportunity. Maps are generally not provided. Adventure locations (like dungeons) are not provided. For me, this is the meat-and-potatoes of GM preparation. I should add that the Het Thamsa adventure we played is a great improvement on the other TTI booklets, as far as gameable content goes.
Cave Maps are confusing. The first part of Reach of the Roach god features a series of caves. The caves are connected to one another underground. Each cave map is given separately (although some of the caves are unmapped, like Horsehead Cave), and then a diagram on page 43 indicates how they join together. This ought to work fine, but it doesn't. To indicate how the maps join together, they sometimes use the room names and other times use the map name. I had to redraw it. Also, the main boss is in a part of the complex which is unmapped. Players are definitely going to encounter the main threat of the adventure, so they should have provided a map for this location.
Random tables could be longer. Basically every random table is a 1d6 table, which starts to give repeating results quite quickly. I guess they are intended merely as a starting point for the GM to add to. When we were exploring the caves in the Reach of the Roach God, I started to get repeating results almost immediately because each chamber of the cave was a result on a 1d6 table.