The use-case for Simon/Speck are things like IoT and other places where you are not only limited by CPU power, but also by message sizes, while on the other han

The use-case for Simon/Speck are things like IoT and other places where you are not only limited by CPU power, but also by message sizes, while on the other hand the amount of traffic is so small that you can live with significantly smaller Ns in various 2^n attack difficulties than normal use cases. For such applications it is certainly better to use Simon/Speck than rolling you own proprietary algorithm (see KeeLog, NXP "Crypto-1" as used on MIFARE Classic and so on). Probably only other algorithm with similar level of scalability which was openly peer reviewed is RC5. In comparison to Speck/Simon, RC5 is significantly more complex (due to it's key schedule) and is not constant time on platforms without barrel shifter. (X)TEA and such things can be similarly tuned for various block and key sizes, but there are no recommended parameters for doing that and thus you get into the "rolling your own crypto" territory very fast.