I've run into quite a few issues - I think that most of them have to do with page updating. I don't have time to go through the entire interface now (nor do I care to, unless you want to send me a check, haha) but here's a few that stood out right off the bat:
Turn on a zone manually: clicking shows a "Sending to device" popup frame with a "busy" spinner - it never goes away. I can move mouse off that popup and the popup goes away, but if I reopen it the spinner is still there. I have to manually reload the entire page to get rid of it, which seems to suggest that the actual device communication did finish a while ago, but the page didn't update accordingly.
Manually running a zone: (related) the page doesn't update to show the "watering now" status unless I manually reload it myself using the browser's reload button. This is particularly problematic since the updated page has the only interface I could find to abort the watering, so if you manually water and want to stop it, you have to reload the entire page yourself.
Add a watering schedule: select fixed interval schedule - on zone selection popup all available zones are selected - none of the checkmark buttons seem to work - I can click on any one but nothing changes visually - the zones may indeed be getting deselected in the code, but in the interface they stay checked - I would assume the intended behavior was for them to update to visual selected/unselected state. Clicking Next then Back in the wizard doesn't fix the visuals.
...those are but a few...
I used Chrome Version 58.0.3029.110 (64-bit) for the above, but if memory serves I was seeing the same issues on Internet Explorer and Firefox.