As long as it grades 1/4" per foot you will be fine.
If you leave a clean out above, you're golden.
I'm doing a remodel with all-new plumbing system. The design I am implementing has some long (~30') horiztontal runs for 2 lavs, a laundry sink and laundry standpipe. These runs are in the wall for about 8' then drop down to joists below and run an additional 20' or so horizontally before dropping down the waste stack. Along the way, a tub, 2 WCs and an additional 2 lavs join in before all going vertical down stack. Pipe is 2" for first 16' then ratchets up to 3" and then 4" as other fixtures join in.
The reason we are going horizontal has to do with a lot of engineering of beams was done in basement. Dropping down to basement from various fixtures would require routing thru these beams and we didn't want to penetrate these nor did we want to lose the ceiling space by running beneath.
So, I believe everything is to code... but will the long horizontal run cause problems and if so, what will those be?
Don't forget that most fixtures have to be vented.