Question: Is USB a reasonable choice for a local bus?

what are pros and cons?

what reliability concerns are there?

**Bus spec** device count ~4; peripheral devices are permanently connected to same PCB as host (soldered or via headers).

Some device are puny, e.g. serial port or temperature sensor, some are complex, e.g. an LTE modem or ethernet or wifi. Peripherals are such that are available as standalone USB devices or same chips are used in such devices. Linux is supposed to have all the drivers already.

Ideally I'd use only one bus (with hubs if necessary) for simplicity and eventually modifications without some components and with other low-speed components.

**System spec** ARM, Linux, DC powered, roughly equivalent to an Android tablet.

Answer: 
Unless the device already has a USB interface, such as a keyboard or mouse, this is going to entail a lot of work and extra hardware. For example, connecting to a temperature sensor that only has an I2C interface. You are not going to be able to add a USB interface to the sensor, so you will have to add a small microcontroller, such as a PIC18 that has both a USB device and I2C interface. The PIC then talks USB back to your host and I2C to the temperature sensor. But why not just connect the temperature sensor directly to your host microcontroller?
SPI 

Pros

* good for 50Mhz
* can communicate with multiple slaves
* simple protocol
* Full duplex
Cons
* Not a standard interface

I2C

Pros

* many slow chips uses I2C
* broadcast protocol where multiple slaves can reside on the bus

Cons

* slow speed
* more complicated than spi

USB

Pros

* standard interface
* probably driver ready and tested

Cons

* very complicated to debug
* with hub, the peripherals will share bandwidth

Other interfaces include SDIO, UART etc

Again as already mentioned, you will have to define the bus hierarchy with your bandwidth requirements. Probably the LTE modem would require a direct memory interface or a PCI interface. 

Also there are phy translators like FTD232 (<http://www.ftdichip.com/FTProducts.htm>) which can do USB-Spi, usb-uart etc and is widely used