DL's award availability may not be the best, but suggesting that DL "sucks" because there is no availability to Europe during the peak summer travel time a mere 1-2 months before you want to travel is flat out rEdiculous.
I don't expect that you'll have much luck finding an award ticket to Europe during the summer at this stage on any airline, whether SkyTeam or not.
If you want to use miles to go to Europe during the summer, in the future, the best bet is to book 331 days (to the day, at midnight) prior to when you want to leave.