My 2 cents as current vSphere user (searching for replacement after new VMware prices knocked my socks off):
Tags in XO are currently only "view filtering feature", nothing more.
There is no method in XO to (without selecting each vm separately):
- attach ACL to group of vms
- select group of vms for backup job
Tags are also error-prone, as I have to manually enter tag name each time (for each vm) and possibly make a typo (there are not even "hints" from current tag names).
I would like to add vm to "group" and this should cause deriving ACLs from "group" and adding to backup job assigned to "group", etc; whatever this "group" would be - folder/tag.
As I have ~500 vms, grouping is "must have" for me.
I'll live if "groups" are tags not folders.
But not having this at all in XO is BIG disadvantage for me. 
(without this I quite like what I found in xcp-ng/xo).
For me - something like folder seems more natural for such things than tags, but I am biased (as vSphere has folders...).
And generally - in current situation (new prices for vSphere) I suspect there may be more users like me, searching for vSphere replacement, and making XO more friendly for them would be advantage for them...