My aversion to such systems is primarily motivated by the fact that every one of such system somehow penalized resubmissions. I probably don't have anything against "you have to write program that is compiled by this gcc/llvm commandline without producing any diagnostics and then passes this intentionally partially documented test suite". But in most cases the first part ends up meaning something like "cc -Werror -std=c89 -ansi -strict" where the real definition of what that really means depends on what exactly the "cc" is and the teachers usually don't document that and don't even see why that is required (ie. you can probably produce some set of edge-case inputs to gcc to prove that gcc is or isn't valid implementation of some definition of C, but this conjecture does not work the other way around).