Miter saw: Am I crazy?

It's an old problem with many possible causes. Blunt and or gummed up teeth on the saw blade can cause deflection very easily. Different timbers prefer different rake on the teeth. Some commercial shops use negative rake which tensions the blade in the cut and improves accuracy. Harmonics in the motor and gearbox can cause flutter in the blade especially in harder timbers.
All to often the solution is to reduce your cut rate. Take it easy. A lot of guys are like a bull in a China shop with mitre saws.
Backing block's support the cut better because it stops the piece vibrating as you  cut. Those teeth are like thousands of hammer blows to the timber and deflection leads to imperfections.
Some guys don't even let the saw come up to speed before they cut ! Lol.
But then there are mitre saws that are just plain crap. If you want the ultimate perfection in the cut you can't beat the old guillotines. 
Generally speaking if I need perfect mitres I use my home made cross cut  cradle on the panel saw. Not my compound Makita.
I will use a backing strip clamped to the fence for maximum support.
For small delicate mouldings on Harpsichords I make a mitre box and cut them with a Japanese saw. 
OR make a mitre shooting board and use a low angle block plane if you want perfect.
So many ways to skin the cat. 
Carbide blades are rarely perfectly sharp for more than a few hours after regrinding. They are not as infallible as people think.
I keep one cross cut blade aside just for mitres. It doesn't get used for anything else.
🍻.

Added later 7 h 59 min 43 s:

Ps. Thin blades are prone to distortion under load. Try a thicker kerf blade.

John McGaw (Bodger) wrote:

Dunno. My semi-ancient DeWalt DW716R seems to produce tight miters as long as the blade is clean and sharp. Of course I did do all of the appropriate alignments. Come to think of it it is probably time to check the blade and alignments again since it has been a couple of years now. I wonder if a simple down-up saw like mine is less susceptible to errors than the down-slide-up sort.

Excellent points. Accumulative error is just a fact of life. I didn't mitre the beads on my windows and door's. I coped them to get better moisture resistance. But as in all things there are so many extenuating circumstances and options to consider.
Mitres have a nasty habit of opening up in some applications but when properly applied ? Very satisfying.

Added later 03 min 07 s:

Bill Howatt wrote:

John McGaw (Bodger) wrote:

... I wonder if a simple down-up saw like mine is less susceptible to errors than the down-slide-up sort.

Anything that guides/moves is a potential source of error.

Absolutely 💯.  Keeping guides clean and dust free helps but the more complex the set up the more room for error to accumulate.
Anyway, preaching to the choir here.

When size of pieces permit I use a Lion Trimmer. I keep the cutters razor sharp and it does a great job.