I use a DeWalt 715 compound saw, and it makes perfect miters as a reward for perfect technique. I had to fiddle with the adjustment arc a little to perfect the angles. If I were to replace it I would buy a 716 version, since the upward pointing thing on the right side wing of the 715 tends to bounce dust back onto the fence table, the 716 directs most of the dust away from the table. A lot of framers use the DeWalts and seem to like them. Mine has gone several years with almost daily use, and all that's threatening to fail so far is the switch. The bearings still seem fine. It's best to avoid the sliding style miter saws since those tend to track poorly during heavy cuts. And some people think 10" saws make better cuts, although my 12" DeWalt gives me nothing to complain about.
The back fence is an old Phaedra, which is just a plain old aluminum back fence by another name. It has a convenient stop block, that's its best feature. And there's a very accurate measuring tape affixed. I never applied the fancy measuring decals that came with the Phaedra, they offended my sense of purity. And I can easily break it down and lean it against the wall if I need. The saw itself is supported on something like a 1950's TV dinner tray on steroids. Each of the two fence wings is supported by something like a two-legged tripod. The thing is so amazingly accurate for how it looks!
The blade matters a lot. The 100 tooth "framers saws" are probably not optimal for DeWalts and other "construction site" saws because they are intended for constant rotation on traditional double-miter saws and will oscillate for a while after the sudden accelerations you get with the construction site saws, and also during the cuts. ATB Freud "Diablo" finishing or cross-cut blades from Home Depot in the 72 to 80 tooth range make beautiful, smooth cuts in wood, and Freud 72 tooth TCG blades are perfect for non-melting cuts in polystyrene. But use a blade intended for your DeWalt or whatever, not an official "framer's" blade. The blades you want are designed to be stabilize quickly after those abrupt accelerations, and they are also optimized for the higher RPMs you get from the DeWalts etc, rather than the slower RPMs of classic double miter saws for framing. That difference in RPM is also why the coarser 72 to 80 tooth blades are optimal, because then you have that same "tooth rate" as the slower rotating classic saws with higher tooth counts.
Beyond that, be sure that the moulding is clamped down real good. There is a tendency for the blade to draw the moulding towards the saw during the cut, that's a contributor to miters that don't fit properly, right after warped moulding. Now there's a subject in its own right.
OK HERE IS THE BEST MITER CUTTING ADVICE YOU WILL EVER RECEIVE:
Most of the problems with non-fitting miters occur not because of your equipment or bad clamping, but because the moulding is slightly warped. You can easily overcome the problems caused by warping.
Buy some 1/4 x 1/4 x 36 "hobby wood" from Lowes. Cut it into 3 inch lengths.
When you cut the moulding, be sure one of the those 3" lengths is between the moulding and vertical part of the back fence very near the blade, and a second one is between the moulding and the vertical part of the back fence near where you will later cut the second miter.
Long story short, this keeps the warped part of the moulding between your two sticks away from the backfence, and prevents inaccurate miters due to the moulding rocking against the fence. The two miters will be in perfect relation to each other, or as perfect as your saw alignment permits, while the moulding in between may still have some wiggle-waggle.
The moulding between the miters will still be warped, but the miter cuts themselves will have the proper relationship and when you get to joining that elusive 4th corner you will amazed.
Yes, safety issues with that type of support! Clamp. Be careful. Wear a face mask. And of course a nose and mouth filter.