MB manufacturer didnt even put 32MB BIOS chip when they knew they had to supportfor three generation, you expect them to make their PCI-e 3.0 tracing PCI-e 4.0 level??
Those B450 boards are as solid to to run PCI-e 3.0 requires. Not more not less. Don't forget the price of B450 boards compare to Z370 board.
Also power delivery system is not the same as PCI-e trace. Power delivery dont have data integrity problem that PCI-e has.
I would agree with that and the PCI-E 4.0 tracing. It is special made tracing that needs a more clean signal. Better shielding also. Too much interference if not.
The compatibility part and that is for all...
Its not a vendor obligation to fully support the next and unknown architecture, nor AMDs to reveal what has on plan. AMD promised compatibility and has deliver.
We have seen this before 10 years ago with AM2, AM3, AM3+ CPUs that were all compatible with each other and not only with their dedicated chipsets. But not with all the new features. its the same. Nothing to complain about.
What MB vendors screw up from time to time is to fail to fully support the CPUs are suppose to support. We've seen it with few B450s and 2000 and few X570s and 3000. But I guess some boards are extreme low budget relatively to their line.