I work for Delta so attach any bias you might want.
For a airline the size of Delta we are running the best operation I have ever seen. Our completion rate dwarfs anyone else at better then 99.8%. We are on track to have better then 100 days this year without a single flight cancelation for any reason including weather. All the other major airlines combined will not exceed 20. We recently went over 4 straight days without a single canceled flt! No other airline worldwide approaches that.
Having said all that none of the above applies to the Delta connection flts that are subcontracted. This would be any flt on a aircraft with 76 seats or less. Be very careful with those flts and check the status starting the day prior.
Delta does connect to EIS with Seaborne. Looked at connecting my daughter and granddaughter that way in May. Very expensive on the dates I needed. YMMV