There are some behavior differences in LSO and MONO which might break content with some examples in this link under VM Detection. I am sure there are many more undocumented gotchas too. https://wiki.secondlife.com/wiki/LSL_Hacks.
LSO scripts also seems to be less likely to break due to things like the "log out due to failed teleport bug", which makes them perfect to build a self repair platform from. I have several products that have a little LSO script in them that reboots broken mono scripts. Of course this shouldn't be something we need to add, but it is until some long standing bugs are fixed. Given that LL are not adding new functions to LSO I can't imagine it creates a lot of extra work to maintain, and I would rather they spend the time swatting some long standing bugs rather than re-coding the viewer and regions to take something away which might send the rest of us scampering to fix stuff for years to come.
All that said, it will only really effect long suffering creators who were around before MONO, and probably has some advantage in making server code smaller. I'd support it if it meant scripts would start working the moment an object is rezzed. Scripts used to start instantly until a couple of years ago when a bugfix introduced a lag into newly rezzed scripts. That is the biggest scripting issue I have been waiting for LL to fix. It makes simple things impossible or requires laggy workarounds where temp-rez objects need to be pre-rezzed so their scripts are running, just in case they are needed.