VoLTE has been "broken" (from a standards perspective) since before it even launched in most markets - it is just too "finicky" and configuration-specific. LTE/4G demonstrated the success of standards in that it "just works" as long as you have the right bands in place. Since most (modern) phones have the bands needed for the main global markets, you can pop in a SIM card and get connected anywhere.
VoLTE is far more complex, and there's network-specific configurations "hard-coded" into the (usually) Qualcomm modem firmware, including IPSec parameters for communicating with the IPsec endpoint in front of the IMS SIP server.
An example of one of these configurations (in JSON-ish format): https://github.com/JohnBel/QualcommMBNs/blob/master/extracte... - even including things like a "user agent":
```
"ImsUserAgent": {
"ValueString": "Vodafone VOLTE Qualcomm"
},
```
VoLTE roaming can work, but it's complex and requires specific roaming agreements between carriers. These arrangements need negotiated individually (for now) between operators, and have a number of considerations including technical and legal - since there's IPSec in use, local laws (in the country you roam in) might restrict the ability to do this, as local lawful intercept wouldn't work. The home network needs to set up rules for which networks won't permit IPsec to be used (I believe AT&T will do this, but only where there's a legal requirement to permit local interception of calls).
The long and short of it is that, as you say, VoLTE is a load more complex than it ought to be. Throw in delivery of SMS-over-IP (settings also in the above MBN example) for extra complexity - many operators rolled out VoLTE/VoWiFi without SMS support, and some still don't properly do SMS over IP as far as I recall.
5G more generally is starting to head this direction too though - with the complexity of EN-DC supported configurations for 4G, and the very limited support for 5G standalone, even among devices advertising it, I fear with 5G we will have another few years to go before you can buy a "5G" phone and have any confidence it will work. VoNR (the successor to VoLTE) will likely have all the same issues replay as we saw with 4G.
I really wish we could get carriers and standards bodies to bash heads together and set up a "baseline" standardised IMS profile for VoLTE/VoWiFi - there's nothing inherently preventing this from working, as you can easily use VoLTE on a private network if you follow the standards, but operators love to meddle, tweak, and add custom useragent strings etc.