I’ve been back riding with flat pedals for a good few years now, and prefer them for general MTB rides, rides with some hike-a-bike and especially when riding challenging trails I’ve not ridden before. But do still use SPDs on the road and cx bike and sometimes on the MTB for longer rides when efficiency is a priority.
One thing to note is that I can sometimes find riding modern flat pedals (with modern tacky soled shoes) a bit of a distraction when I need to reposition my feet mid descent. With SPDs, once clipped in your feet are always in the same place. And I’m definitely quicker up and down with SPDs.
Oh, and remember to drop your saddle about 6mm when moving from SPDs to flats as there is a slight but noticeable height difference.
So in essence, both modern flat and spd pedals are great with slightly different strengths and weaknesses so comes down to personal choice.