It was jury-rigged, because it was beyond Federation technology. There's no Federation technology, with or without jury-rigging, that can reliably do this. Patterns degrade in transporter buffers; that's what they do.
Federation technology from 10 years before TOS can potentially transport someone from Earth to Q'onos with only a portable transporter! There are always... possibilities. (And yes, I do think this is an abomination.)
I like to think that it's not so much the pattern in the buffer that degrades (it's just information that cannot be created or destroyed after all) but rather the quantum energy linkages tied to the real person that has been phased into a dimension that borders sub-space where space and time have no meaning. This means that limits on traditional transporters are caused by an inability to maintain a confinement beam (containing the linkages) over long distances. Too many energy linkages leak away over increased distances in our dimension and you can't retrieve enough of the person to survive.
Lose linkages and not all of the person phases back. You can 'boost your matter gain' (per TMP) which effectively uses the scan taken when they first transported to import replicated matter and fill in the blanks but too much replicated matter and the person turns up dead.
In rare transporter mishaps, somebody with 50% replicated matter survives and a transporter duplicate is born! It's also possible for transporter accidents to create epigenetic alterations that can lead to personality disorders (per the Enemy Within and transporter psychosis).
It could also mean that the same crewmen should not use transporters too often or there might be health consequences for having too much replicated matter among your cells at any one time.
So essentially as long as you can maintain or re-establish those linkages, a person in the transporter dimension can live indefinitely as long as enough of them remains there. There is just no known technology that can bring them back.
Since Ilia was not transported but without question transformed into data, if she were to return, she would be a clone of the original.
And I am unanimous in that.