I needed to send an exact amount of swedish kronar (eg 1234SEK) and found it extremely difficult to do via the app.
To start off with, having to enter a random amount in GBP and then tweak it until it was vaguely the correct amount (eg 1234.12SEK) was not fun at all.
Then after entering the recipient’s details it gave me a different (lower!) exchange rate and I had to go back and fiddle with the numbers again until it was right, then go forward and fully re-enter the recipient and find the echange rate had dropped again and so on multiple times.
I ended up sending several SEK more (about 50p) than I wanted to just to cover the random fluctuations in exchange rate between the amount entry screen and the confirmation screen.
After the confirmation screen there was absolutely no indication that a transfer had taken place. The marketing piece said that it would use faster payments but that doesn’t seem to have happened and when I worked out how to log into my shiny new transferwise account it said I need to send them a bank transfer.
As first impressions go this is a spectacularly bad one.
If non-faster payments are being used it could at least mark the money as debited from my account 
It’s been like this since it was first introduced; very frustrating.
Until Monzo sort it out (and they are aware of it already), I’d suggest using TransferWise directly.