The TPMS units are much more complicated. They are both send and receive data. They operate on two frequencies. Take a look at page 29 of the BMW University TPMS document:

 

That was a good read. But I do not think the sensor itself is complex. It sends and receive data yes, but their is no processing being done at the sensor level at all. The TPM Control module processes the data to the iDrive. The sensor only sends a signal at a predesignated interval and when it detects a change in where it was set. It receives data by module requesting it to perform a single task "what are your values". Yes it is more complex than the old monitoring system but not enough to process data from commands being sent from a program. Who knows maybe I am wrong but when I look at the diagram it shows simple signal and data flow.

 

First, the wheel units of the mentioned systems are the same (in general). Only the system with trigger antennas offers more possibilities.
Actual systems (without trigger antennas) are different.

The wheel units do:
- temperature compensated pressure measurement (complex)
- temperature measurement
- acceleration measurement
- automatic mode and power management (complex)
- LF (125kHz) reception (complex)
- RF sending on request (LF), on current mode and on pressure change detection (complex)
- fault detection on wheel unit level (sensor defects, battery, uC, etc.)

They do not supervise warning levels. This is done in the ECU.

You always want to keep the signal flow as easy as possible. This isn't an indicator for the complexity of the wheel unit.