1. Tooling integration is a PITA, especially with Visual Studio.
2. Nothing but trouble merging the stored output from the tooling if you are in a branchy development. Basically, merge the CSS, then run sass/less and check in the change. Too much friction on a large project, especially when it comes to a non-Sass/Less dude doing the merge.
3. Parsers are both shit. Sometimes attributes don't get "understood" properly resulting in compile errors and point blank refusal to work.
4. Hacks used for older browsers aren't parsable by either.
5. It's another layer of abstraction. Less abstraction decreases debugging effort.
Sorry but I don't buy it despite the promoted advantages.