What are the draw back of having multiple smps stages in series?
$$\color{red}{\boxed{\text{You are focusing too much on efficiency as a parameter}}}$$
Consider the stage 1 circuit that you say can produce 9 volts at 2 amps with 92% efficiency. Consider how much current the stage 1 circuit needs to supply stage 2. Stage 2 delivers 100 volts at 10 mA as per this: -

output - 100V 10mA output @ 87% efficiency

That's an output power of 1 watt and it means that your input power will be 1 watt ÷ 0.87 = 1.15 watts. That's an input current of 1.15 watts ÷ 9 volts = 128 mA.
Now, going back to stage 1 - it is stated that the efficiency is 92% when delivering 2 amps. However, when delivering 128 mA that efficiency is going to be much, much less. You can make a broad assumption that 50% of the losses when delivering 2 amps will still be present when delivering 128 mA. The losses in stage 1 delivering 2 amps are 18 watt × 0.08 = 1.44 watts so, 50% of that loss (0.72 watts) will still be present when delivering 128 mA.
So, rearranging things, stage 1 is delivering 9 volts at 128 mA = 1.15 watts and the overall input power to stage 1 is 1.15 watts plus 0.72 watts = 1.87 watts.
This makes stage 1's power efficiency 1.15 watts ÷ 1.87 watts = 61.5%. Hence why I said this early on: -
$$\color{red}{\boxed{\text{You are focusing too much on efficiency as a parameter}}}$$
The likely overall efficiency is going to be around 50% based on \$\color{red}{watt}\$ you have told us (the red spelling is my attempt at humour).

What are the draw back of having multiple smps stages in series?

There are no real drawbacks in this case-example except you can probably achieve a better efficiency with a tailored flyback switcher design (circa 80% given your load and input voltage supply). Now I'm focusing on efficiency but justifiably so.