The fact is that the Trezor is really not built on a secured microcontroller, however:
- the possibility of security holes does not mean that there are some
- and more importantly the use of secured microcontroller does not guarantee the absence of security holes
You forgot to add one important issue for Bitcoiners:
- the entire development toolchain for Trezor is open source, whereas dedicated secure micro-controllers frequently have only proprietary toolchains that cost money, are closed source and may require signing onerous non-disclosure agreements and other encumberances.