Easier but slower, especially for large files. I'm not disagreeing but files over ethernet/wifi will be slower then USB. I used to use a NAS for similar reasons, but then found it better for me, to use an external drive. True it can only be used by one machine at a time, but for my usage that's all I needed.
Depends on the NAS. I've used older NAS's that were only able to do around 45M/s. I'm still using an older Drobo 5N that only gets around that. But I've also got a 10 bay ASUStor that will do 80M/s+ and I've built a 15 bay OMV box that gets 90M/s+ (on old/slower drives). For reference, 1G ethernet maxes out at about 110M/s
With USB-A 3.0 external drives, I've seen as low as 30M/s up to a high of about 90M/s. There's a lot of variance there. However if you want to get your full drive speed, USB-C and/or TB3 will allow that (150M/s - 250M/s for a modern drive)
Personally, I'm looking to build my next NAS to handle 400M/s transfers plus move most of my basement lab over to 10G Enet. Transferring 10's to 100-200G at a time over 1G Enet isn't that bad... but it's not the best. However those multi-terabyte copies are a real pain