a) it increases energy cost
b) accessing storage over smb/nfs is not as fast and can lead to performance issues.
c) in terms of workflow, I find that having all containers (I use rootless containers with podman as much as possible) running on the NAS that actually stores and manage the data to be simpler. So that means running plex/jellyfin, kometa, paperless-ngx, *arrs, immmich on the NAS and for that synology's cpu are not great.
In general, the most common requirements of prosumers with NAS is 2.5gbps and transcoding. Right now, none of Synology's offerings offer that.
But really the main reason I dislike synology is that SHR1 is vendor locked behind their proprietary btrfs modifications and so can only be accessed by a very old ubuntu...
As soon as my Synology dies I'm replacing it with Unifi. I don't want all that extra software with constant CVEs to patch.
I am not necessarily disagreeing with you but context is important. I've had 918+ and 923+ and the cpu has idled through all my years of NAS-oriented usage.
Originally I planned to also run light containers and servers on it, and for that I can see how one could run out of juice quickly. For that reason I changed my plan and offloaded compute to something better suited. But for NAS usage itself they seem plenty capable and stable (caveat - some people need source-transcoding of video and then some unfortunately tricky research is required as a more expensive / newer unit isn't automatically better if it doesn't have hardware capability).