I'm not surprised to see this topic come up again, now that you cannot directly publish projects to YouTube...
Online video services (Vimeo and others included) have restrictions on the amount of storage you can use for your videos. You are, after all, using *their* service for distribution. Professional accounts have different conditions than "regular" accounts and you can make use of uploading ProRes masters to YouTube and expect to have perfect results.
The rest of us, however, have to find a way to live within our "means".
YouTube will "accept" pretty much anything you upload to them, but if it's too big, they're going to re-encode it to make it fit. Their algorithm is not the best and you get soft or blocky (jpeg artifacts) video in many cases*.
File size is often related to Project Size (e.g., 1920 x 1080) and bandwidth (MB/s - megabytes per second). Compression is used to make it all fit (mp4/aac -- or a variant of that because YouTube is owned by Google... it might be WebM [VP8 or VP9] — whatever it is, it's an open source version of something). I don't know for sure and I don't care. I only know it is not as good as the compression you can get with your Mac using another app because Apple provides us with a bona fide H.264 codec (and it took them a long time to negotiate that deal! Literally years.)
FCPX always encodes in the highest / best quality range (regardless of codec). There are only generalized options available, but no way to fine tune the encoded video it produces. You need a second app. I personally still use Quicktime 7 Pro because I haven't moved on from Mojave yet (but the end is near... drat!)
YouTube (last time I researched it) states a bandwidth limit of 8 MB/s (or 8000 kbps -- same difference -- for 1080 -- other video sizes get more, but it's relational...). If you upload a .mov (h.265/aac) file that comes under the bandwidth restriction, YouTube will *NOT* re-encode it. You get what you uploaded. QT7Pro does a pretty amazing job getting video down that small and still look really good. I've never had any luck with Compressor (although admittedly, the last version I used was v3) and I never liked it. There are just too many settings options to screw up.
Lately, (in the last couple of years... maybe) YouTube seems to be "relaxing" that hard limit. I've had success uploading 1080 h.264 up to about 13 MB/s and getting what I uploaded. That extra 5MB/s is ***considerable***! And, the last *several* versions of FCPX has been generating h.264 "masters", best quality (2-pass) at under 15 MB/s (in general) averaging around 12.5...
So, my recommendation: export your projects from FCPX as H.264. Open in QT Player and get the Data Rate stat before you upload. If it is roughly 12.5MB/s or less, you're probably "golden". If it looks like you need more compression, send (from FCPX) to Compressor and tweak. [As an option, you can try to encode for Apple Devices. You have an option for Faster Encode (not recommended), but it may be that FCPX will compress a little more than the "Master" quality.] (Anybody know for sure??)
Get the bandwidth under the limit -- you'll save time uploading to YouTube, you'll save time NOT having YouTube re-encode the video and you should get much better results. [You can tell if youtube is re-encoding or accepting your video straight out: when the upload finishes and it accepts your upload, it's usually finished "publishing" in less than 2 minutes. If you have to wait longer than that, chances are good it's getting re-encoded.] If your video is such that it requires a higher bandwidth than I've been discussing, you will need to look into self-hosting if possible, which might require a CDN (content delivery network like Cloudflare or Amazon) depending on expected viewer demand, or upgrading to a pro account.
*If you really want to see how YouTube can mangle a video, use the Halftone effect**. You need at least 28 MB/s to make it look halfway decent and about 40-50 MB/s to make it look "perfect". YouTube won't come anywhere near that tolerance...LOL.
**Best practice, duplicate your clip(s), apply Halftone to the top layer and set the Compositing > Blend Mode of the clip to Overlay. [It's a rather decent quick and dirty "cartoon effect"].
[Direct upload to YouTube **never** made any sense to me... but there it is... I think earlier versions of FCPX exported higher bandwidth assuming "pro accounts"... every time I uploaded directly to YouTube, I could always tell the video was being re-encoded. Terrible results... Just wasted my time...]
