I have issues with both... well all. I will say that for a while I had an excellent service with Royal Mail until the postie left and they got in new ones who are either lazy or don't have a clue on what they are doing.
People come and go in Royal Mail, sometimes you get decent workers and other times you get incompetent lazy ones that scan your items off as delivered regardless of where they have been posted too or dumped in some cases.
What tends to happen in my area is that there will be a decent postie around for 2 or 3 months then that one leaves and a new one starts or they have a few working in the area and then the trouble begins again until another postie takes over who actually delivers mail correctly.
Evri are terrible but in most cases they do end up delivering whether that be a late delivery or one on time you still get your things 9 times out of 10 and they take a photo of your door/door mat so if it does go to the wrong address you can find it by the photo of the door.
My main issue with Evri is the way they treat peoples parcels, more than half of mine arrive in semi crushed boxes or torn packaging and if you order anything glass or fragile, forget it... it wont be in one piece.