I don't know the ins-and-outs of btrfs in detail, but having dug into other systems that offer redundancy-over-uneven-device-sizes and assuming btrfs is at least similar: with your two drive example you won't be able to write another 50gb to that array.
For two devices, 1x redundancy (so 2x copies of everything) will always limit your storage to the size of the smaller device otherwise it is not possible to have two copies of everything you need to store. As soon as you add a third device of at leats 100gb (or replace the 100gb device with one at least 200gb) the other 100gb of your second device will immediately come into play.
Uneven device size support is most useful when:
♯ You have three or more devices, or plan to grow onto three or more from an initial pair.
♯ You want flexibility rwt array growth (support for uneven devices usually (but not always) comes with better support for dynamic array reshaping).
♯ You want better quick repair flexibility: if 4Tb drive fails, you can replace it with 2x2Tb if you don't have a working 4Tb unit on-hand.
♯ You want variable redundancy (support for uneven devices sometimes comes with support for variable redundancy: keeping 3+ copies of important data, or data you want to access fastest via striping of reads, 2 copies of other permanent data, and 1 copy of temporary storage, all in the same array). In this instance the “wasted” part of the 200gb drive in your example could be used for scratch data designated as not needing to be stored with redundancy.
