I have XRGB-mini Framemeister and I have connected Atari ST via this device to my monitor.
I also have two cheap, Chinese SCART to HDMI coverters. The difference is huge. Framemeister is really expensive, but if you need almost perfect picture quality, then buy It.
The biggest difference is deinterlace logic. All cheap converters treat input video as interlaced (480i or 576i), but ATARI uses non-interlaced output (240p or 288p), so they always put video through some algorithms to make one picture out of two. They also have sofisticated algorithms to eliminate horizontal odd/even lines. This means, that static picture will be shown more or less ok, but moving will have strange artifacts. Even around moving cursor you will see some "strange" things.
The Framemeister can distinguish those formats and it treats ATARI RGB output correctly (as 240p or 288p respectively). Also still video is more clear and sharp on Framemeister, but even though it is not absolutelly perfect. In terms of dot clock ATARI output is out of spec. In practice it means that some pixels are little wider and some little narrower (at least in medium resolution). It looks good, but if you are real perfectionist, then you will notice that.