Anyone building from source has the ability to potentially including some modifications. Unless you are able to verify that checksum published (by whoever builds the code) "matches" the source code (by reproducing the build) you cannot be sure there aren't any modifications. Published checksums are often meant only for verifying that binary you got matches the original binary. Not for verifying that it is build from specific source code.
Many Debian packages already have reproducible builds, but not everything.
From Debian wiki:
> Reproducible builds of Debian as a whole is still not a reality, though individual reproducible builds of packages are possible and being done. So while we are making very good progress, it is a stretch to say that Debian is reproducible.
But the point is that this has nothing to do with the type of an open source license. GPL doesn't guarantee you that a build is reproducible and MIT doesn't prevent you from having reproducible builds.
Many Debian packages already have reproducible builds, but not everything.
From Debian wiki:
> Reproducible builds of Debian as a whole is still not a reality, though individual reproducible builds of packages are possible and being done. So while we are making very good progress, it is a stretch to say that Debian is reproducible.
https://wiki.debian.org/ReproducibleBuilds
But the point is that this has nothing to do with the type of an open source license. GPL doesn't guarantee you that a build is reproducible and MIT doesn't prevent you from having reproducible builds.