Running a vehicle on methanol as it only fuel source will cause damage to an engine over time. Methanol is more corrosive to seals and metals then other fuels.

However, in the case of methanol injection spray, you are only using it in conjunction with gasoline for short bursts of time so it doesn't really become an issue.

The only con's to running a water/alcohol injection system is the initial cost and the chance for the system to fail. If your system fails and you have no way of detecting it, your engine will fail as well in short time.

Also, you always need to run the same mix of alcohol to water ratio that you were tuned for. If you were tuned for 100% methanol (alcohol) then it's even more important to only use methanol, because its at that point its a secondary fuel source.

 

Listen to codean. I ran meth injection on my old DSM and put 15k on an engine that was 18 years old with meth and had no ill effects even on that and I was running windshield fluid in it which isn't as pure as water/meth mix (maybe I could just see the mineral deposits? I've seen no trace of anything with my current set-up and have checked about every 5k miles). Make sure you're using the same mixture you were tuned for.