The m17n team appears to be hard at work insuring that all scripts defined in Unicode are supported. Notice that m17n has support for both Khmer and Myanmar, as well as Thaana and Sinhala. I could not find Syriac listed on the m17n web site: I wonder if this is just an omission.
The m17n web site provides a demo where you can submit a sample of text to be rendered on their server. This is a good way to prove to yourself how well a script is supported. (My only disappointment using the web demo was that the web application does not let you choose fonts, as some of the default fonts used by the web app are not good. For example, Thai is rendered using the Bitstream Cyberbit font which has typographically incorrect placement of certain vowels and tone marks --perhaps they should take a look at my Unicode Font Guide for Free/Libre Open Source Operating Systems1).