YouTube and Vimeo recompress every video they receive, even on the "best quality" setting. That's necessary on their end (streaming at scale, bandwidth), but it degrades what a client actually gets: banding on gradients, smoothed-out detail, colours slightly off from your original export.
The problem isn't the platform, it's the use case
YouTube and Vimeo remain excellent tools for public distribution. The problem shows up when you repurpose them for a private client delivery: the client compares what's on screen with what you showed them during the edit, and the difference is visible, especially on sky gradients or dark scenes.
Deliver the real file, not a recompressed stream
For a validation round or a final delivery, the client should receive the exported file as is, not a version run back through a third-party platform's encoder. A delivery page that hosts your export directly (a well-tuned h.264 or h.265 MP4/MOV) avoids that double compression.
xFer lets you drop your export directly and deliver it on a page in your brand, keeping the quality you chose at export, without routing through a public streaming service.
Save the double compression for public sharing
Once the delivery is approved and the edit locked, publishing on YouTube or Vimeo for public distribution still makes perfect sense: at that point, the platform's compression no longer affects your client relationship, it's just serving the broadcast.
In short
Keep YouTube and Vimeo for public distribution, not client delivery. A file delivered directly, without routing through a third-party encoder, avoids the compression artefacts your client would eventually notice.