The limit in legislation is 150 EUR 
Monzo have a £100 limit because our cards also support a small amount of offline usage, and this is to prevent you from going over the limit while offline (which we would be unable to see until the next day). We felt it was a much better trade-off to continue to support this case, and make sure you can’t exceed your contactless limit at the same time.
Keep in mind that mobile wallets (Apple and Google Pay) are authenticated already, so they don’t need to use this entire mechanism either