JOOI, is there a range of board.. complexity or whatever? Here you imply a regular PC motherboard is simpler than a server-class motherboard, so I am wondering if there is a workstation-class motherboard which would be, presumably, more complex than a 'regular' PC but less so than a server. An earlier response suggests not, so it would be instructive to know the reality.
In my opinion, the most complicated part of PC motherboard is RAM interface. So the more memory channels your CPU have, the more complex layout is. The reason is that each channel is over 100 traces with very strict rules as to how they need to be routed (length matching, spacing, etc.). All other typical PC interfaces are now serial (SATA, PCI Express, USB, HDMI, DisplayPort), so they are much easier to route because there are less traces, and they don't need to be length-matched to the same standard as RAM traces are.
With that said, there are HEDT (Hi-End DeskTop) CPUs out there which have 8 memory channels (AMD Threadripper PRO), while there are server CPUs with less channels than that. So it's technically possible for workstation MB to be more complex. Also there are workstations which utilize server-grade CPUs (Intel Xeon CPUs can be found in quite a few of them), which further blurs the line.
But then I made a statement about complexity, I assumed a typical mainstream systems with CPUs that have dual-channel memory controller, compared to the open source server system I linked above that has 2 CPU sockets each having 6-channel memory controller.
I suppose what is complex and what is not is somewhat subjective depending on the kind of boards a person deals with on a regular basis. In my case a typical board has an FPGA on it, x16 or x32 DDR3 memory interface and a bunch of peripheral chips (stuff like Ethernet, ADC/DAC, and other application-specific devices), plus power delivery system for all of that and a system controller MCU which orchestrates everything and makes sure all hardware on board works properly. I don't consider such boards particularly complex, but I imagine that many others will consider boards with 300-400 or more components on them quite complex. Even the very simple FPGA board that I designed in KiCAD for beginners (it's linked in my signature) has 169 components on it, and I regularly receive comments saying it's too complex - despite it's being pretty much as barebones as practical.