Default search options frequently only display a handful of possible flight options... and even sometines one or none even though dozens of others are available. I can only find these by selecting search by schedule.

Search by schedule is broken:
Picking times doesn'often doesn't actualy display a flight that is within 30 min of the time selected. Have to choose a time a couple hours away to get the desired flight to appear
Selecting fare type 'discounted coach' doesn't work, always returns lowest fare.
Using book by fare bucket doesn't work, often either returns errors or just restarts the search at default price options when you select the last flight of the itinerary if it is multicity.

Award tickets are broken:
Simply clicking on the 'change flights' of an award ticket will cause it to be canceled even if you do not follow through with making the change or exit out. The flight vanishes from your itineraries and you don't get the miles back... a call to delta results in the agent saying the flight was canceled by you and then numerous back and forth and time spent to get it reinstated.

P fares broken Amex reserve first class companion cert:
If you search for a flight by class and select first/biz and it comes up with P fares instead of A (which is quite common now since DL started selling P), then trying to use a companion ticket results in the system returning fares that are around 2x the price of the P fare which makes the companion cert worthless.