Actually ST requirements are 100mA for lighting and 30mA for plug points (power points for equipment easily accessible by hand). In this case, 100mA main RCDs are acceptable, provided that you have 30mA to cover the plug points in addition to the main RCD. If there is only main RCD it has to be a maximum of 30mA, but ST doesn't really recommend 1 RCD for the whole DB.

Of course, this is in addition to water heater circuits which are required to be protected by 10mA RCD (dedicated).

Adding a 10mA RCD in cascade with a 30mA/100mA is not an issue. The device is meant to trip the power due to safety reasons, what may happen is that both may trip at the same time if there is a problem, which is more preferable to it not tripping when the problem occurs.

Doing it "flat" is of course preferable but will involve alot of work at the DB, which is dangerous if not done properly.

Both may still trip, due to tolerances of the device itself especially when 10mA and 30mA are quite close in rating. There is a proper way to do discrimination of tripping for the devices, but even without the discrimination it's still preferable to put them in cascade rather than not having sufficient protection.

Physical installation of electrical circuits can get quite misleading. If the cables are terminated in a daisy chain as per your picture, that is a parallel(flat) installation instead of cascading. Because the looping is happening at input of RCD only.

For example in below case the RCD are separate and will only trip 1 RCD if a fault happens at A / B / C / D respectively.


But if cascaded like below any fault in B may trip both RCD, but a fault at A will only trip the bottom RCD.


It's considered cascaded when output of the RCD is connected to the input of the next RCD, as you can see in the picture above.

In your case, if all 4 RCD are installed in a cascade, it's not dangerous but likely to be redundant because any fault at the end of the RCD will likely trip all of them, unless there is sufficient discrimination between them. RCDs are protective devices, so they don't introduce danger through their operation. But if RCD is not installed or is faulty, then the lack of protection from the RCD can be a hazard instead.

Actually devices that can cause safety hazards like water heaters already come with built in RCD nowadays, so it's not necessary to cascade multiple RCDs (as long as all DB circuits are properly protected already with the correct ratings).
Rather, it's recommended (and also more important) to test the RCD regularly instead to ensure that protection is intact. Some water heaters even allow you to test their RCD.
Because like what ceo684 has mentioned, the RCD will fail someday, and the only way to find out is by testing them.

Actually ST recommends monthly testing for RCDs, which i believe most people do not and will not do due to the hassle.
But if someone were to practice it, having all the RCDs in the DB would be an easier task than testing DB and all water heater switch locations. So the recommendation to have them in DB rather than at the switch is a sound one.

RCD is just a name for a class of devices that protect people against earth faults. What you probably mean is RCCB which are common used electrical devices.
Higher range devices are ELR and EF relays, which are overkill for domestic use. (for example RCD = vehicle, RCCB = car, ELR = lorry, EF = trailer)
That said, water heaters should be using RCD nowadays since ELCBs are no longer allowed due to obselete technology(voltage based) compared to RCDs (current based).