It is not a fact that IPIP is difficult. It is either easy (if your ISP router will forward IPIP or at least allow it through a DMZ) or impossible.
This isn't the fault of ipip, it's the poor service of the provider in this case however for decades, as ISPs have put up hurdles, we as hams and inventors have found ways around what ISPs to do stop our routing.
routing IP datagrams over radio links
This isn't the optimum way to do IP over ax.25 based radio links. Datagram is like UDP - lacks error correction. Case in point would be VoIP/SIP in a weak area. Missed frames stay missed and broken in a stream of data. Things get grunged.
It doesn't need any special routers, renumbering you network or any specialist network knowledge.
... and the ham doesn't gain knowledge but continues to be an application user, then when something happens due to changes by their ISP they don't know how to properly debug it because they haven't gained the skills needed to debug networking.
The system we use has dynamic routing by default since IP on VHF/UHF is encapsulated under ax.25 very similar as if we were able to have BGP based border routers between us. If an ax25 path breaks, and an alternate path exists the IP based socket stays open and connected at both ends. We're also running a higher MTU than a netrom (MTU of 192 bytes) based system does without fragmentation.
This is quite impossible with the software solution previously mentioned.