> It doesn't support -B
One of the mottos of the project - "you'll never need to do clean build". Things like "clean" and -B are used in case if build is broken by incorrect dependency information (esp it happens often for recursive make). Tup provides "build consistency" - the output is always correct because dependencies are always correct.
> make -n
> it doesn't seem to support out-of-source builds
AFAIK the tup author in favor of adding it. It is better to contact the maillist as I am not sure about his plans.
Oh, yeah, I remembered 4th thing that I like in tup - "buffered output". Output from commands is always printed atomically. No more interlaced output! The interlaced output is especially annoying if you have an error in one of widely used header files - this makes error messages really difficult to read.