I think demons and devils are probably the creatures I use the least in my campaigns. I grew up none-religious, so I have less of a connection with these monsters. If I use a devil or demon at all, I usually only hint at it, but never reveal it. I feel a demon or devil loses some of its menace if the players can simply fight it directly. So to maintain the dread of something so evil, I simply describe the evil, but never have them face it directly. One of the big evils in my campaign is a demon queen that rests somewhere at the bottom of a giant crater that radiates evil. I make sure no one ever wants to get near to it. And if they ever were to face this creature directly, the campaign would simply be over.
I am guilty of using demons the most in my campaigns and not because of any religious connotations, I could care less about that. I do think your opinion towards demons and devils is skewed do to a misunderstanding of the roles of the two creature types though.
1st Comparing Demons and Devils to each other is like comparing night and day, I realize that that is a prevalent thing among players and dms. On the surface they could be said to look similar (although not really), they are both fiends, and they both occupy the lower planes.
2nd Devils are the Machiavellian tempters, they plot over the course of thousands of years. I'm of the opinion that in most cases player characters shouldn't be interacting with devils unless it suits the devils needs or is part of some plan. Why would a lvl 7 wizard be able to get over on a 30,000 year old plotter? Devils need to be unpacked carefully over the course of a campaign to get the full effect of this creature. If the pcs are just encountering devils as enemy's to defeat, the dm is using them wrong.
3rd Demons in d&d are basically a cancer on the universe. Demons are generally the most feared creatures in all of the planes. Why? Because the Abyss is a living plane that spawns infinite screaming horrors whose soul purpose is the corruption and destruction of everything in the universe. Demons are the most numerous creatures in all of the planes, because the abyss spawns them infinitely. When a portal to the abyss is opened demons pour out of the portal like a dam just burst, screaming, killing, burning and devouring whole all they come across. One of the biggest disasters a country can face is a portal to the abyss being left open and unchecked. Everyone would quickly be overwhelmed if it wasn't dealt with. Gods have fallen to hoards of rampaging demons. Gods....
Demons are scary because they are horrible destructive evil creatures. And not evil in the way that a red dragon is evil either. Red dragons want to throw their weight around and flex their power. Most demons don't give a damn about money, or anything that most mortals will care about, they just want to get into your plane of existence, flay you alive, and devour your soul (and they don't need a pact to do that, they just take). And thats not even getting into what the demon lords are about.
Its important to remember the differences. Devils are about law, rule, hierarchy, and seducing mortals into doing what they want whether that be entering into a pact, assassinating a king, or whatever. At the end of the day Devils believe they deserve to rule the universe because it is their divine right.
Demons really don't give a f*** they want to destroy everything, twist everything , they don't want your money, they don't need your allegiance, although they delight in twisting mortals, they just want to burn the whole thing down.
Devils and demons hate each other for these reasons. So lumping them together, shouldn't be done, completely different.