I used a splitter because with an M2 NVME installed it cut down available lanes and solved the problem quickly avoiding any more down time but the M2 NVME makes compiling drivers so quick i can do in an hour what used to take all day when testing driver versions....
It makes sense to use NVME for a test rig but a production rig would be better to use SATA to free up
PCIe lanes for GPUs.
Mobos with more features that aren't needed for mining like Bluetooth, high speed USB, extra SATA ports, Wifi, NVME, etc,
will have fewer lanes avalable for the PCIe slots.
In the case of a budget CPU like Celeron or Pentium, it may not supply enough PCIe lanes to serve all the slots
at once.
Check the mobo and CPU specs before buying.