OK, for the purpose of this post I'm going to be calling front rails that fit on between a delta ring and a handguard cap RIS, and front rails that fit on a replacement barrel nut and float free at the front sight end RAS.
(1) RIS. As far as I can see, there are two styles of RIS, (a) those that fit on JG, Echo 1, TM and SRC models, and (ii) those that fit on just about everything else (G&G, G&P, KWA, ICS, CA, etc. etc.). These latter use a "real steel" style delta ring assembly. Most but not all after-market RIS are designed for (b).
Example of (a): http://www.airsoftgi.com/product_info.php?products_id=2371
Example of (b): http://www.airsoftgi.com/product_info.php?products_id=4593
(2) RAS. Typically to install, you need to undo the barrel nut and screw on in its place some kind of fixture to which the RAS then mounts. Compatibility issues occur when the thread pitch on the new fixture that replaces the barrel nut does not match the pitch of the threads on the front of the receiver. Brands of AEG with possible compatibility issues include G&P, KWA, and (?) ICS.
I have read posts where people simply remove all the paint from threads or do some gentle dremelling into order to achieve a good fit. Otherwise, there is a special tool that can be used on the receiver's threads to regrind them to the usual standard. For example: http://www.airsoftgi.com/product_info.php?products_id=4264
Presumably, G&P front sets fit on G&P receivers just fine, but what I'm less clear about is whether the compatibility issues work both ways, that is, are there any problems in putting, say, a G&P RAS on to a standard receiver's threads (G&G, CA, Madbull, whatever).
Other questions: are there any other brands than the ones I've mentioned that have the kinds of compatibility issues I've described, and are there any other kinds of compatibility problems for RIS or RAS that I have not mentioned?