As a maintainer of a >130kloc Haskell codebase spanning thousands of modules and hundreds of packages, I avoid TH primarily for one reason: slow builds. Painfully slow.
For cases where GHC.Generics is an option, I always take GHC.Generics over TH. Yes, Generics can be slow at times too but this somewhat controllable with various GHC_OPTIONS. Either way, there is no loader and interpreter overhead... and a lot of Generics code compiles in less time than it takes TH just to load `text` or `aeson` ;-).
Other reasons:
* Some TH libraries add implicit dependencies that are not tracked by the build system
* GHCi/TH's loader is different than the OS loader, sometimes causing failures if a TH dependency touches C++ libraries
* No cross compiling ability... not an issue for me but it's painful for those targeting Javascript (via ghcjs), ARM or smaller x86 embedded systems
