The problem I've found with all hlsl settings is that the aperture pattern is not perfect. I don't know why. You can see this issue, for instance, in the green neogeo loading screen. It's close to be perfectly homegeneous, but it's not. In games this is not noticiable. The big problem comes with vertical scrolls. Try, for instance, Willow, 1st stage, the scroll from the clouds to the ground. Don't you see an ugly effect on the scanlines? My screen res es 1920x1080 and I've disabled the screen curvature, if that matters for the configuration.
This problem doesn't happen when using an effect .png file. Actually, that's my solution (I have a custom effect with scanlines and the vertical aperture pattern) combined with SweetFX, to improve color, sharpness, bloom, etc. I'm linking a picture to show how I see Willow with this setup. I've tried not to be extreme with the settings, because too much hdr or bloom, although it may look impressive, makes you loose darker or brighter tones and details.
Another problem I have with hlsl is that it darknens the image and you can't see the darknest details. I can increase gamma in MAME.ini and that fixes the problem, but I have many custom inis with reduced gamma for games with wrong gamma, such as Outrun, Golden Axe, Violent Storm, and these would be too dark. Any solution? Isn't there a way to tweak the red, green, blue ratios to increase brightness globally?