The democrats hate it because of the position it puts them in. They come out looking like the bad guys no matter what they do now. They wouldn't pass a stimulus bill without a crap-ton of their own progressive wishlist items attached, and were bickering over those details while Trump swoops in and does an end-around with an executive order.
BOOM. He gets to blame the democrats for "holding the American people hostage" over political wish list items. And if they come out too strongly against the executive order, he also gets to blame them for wanting to stop a stimulus payment to help "needy Americans hurting from the tragic effects of COVID-19". Etc.
No matter what side of the aisle you are on, you have to admit that Trump knows how to play the game, and the Democrats have been underestimating is ability to out-maneuver them most of the time.