I think that this question should really be reframed as what Roman soldiers were coming from, what they went to after their discharges, and how that turned out for them. As written, there's not a ton that can be said: we don't have the evidence. From epigraphy we know that some discharged soldiers in the Principate did fine after they left the army, and of course many veteran colonies flourished and became cities along the frontier, which were often the bases for medieval cities on the same sites. But surely for the many soldiers that succeeded there were many that failed. We just don't have the evidence.

What seems to make more sense is to look at how the lives of soldiers changed and whether we know of friction resulting from discharged soldiers. For the former question, the answer is quite clear: not much. Military recruitment was always much heavier in the country than in the city. Urban recruitment in fact seems to have been exceedingly rare, reserved even in the Principate for emergencies when manpower needed to be hustled up quickly. There's a little bit of an issue as to whether we actually think that the property requirement was ever formally waived (there seems to be no evidence) but that's more or less beside the point. Rural recruits were always favored and made up by far the bulk of the army, and from early on in the Principate Italian recruitment generally declined--something like 7% of the army was from Italy by the time of Trajan. This means that most veterans by far came from farming backgrounds. Soldiers that received land grants (which, we should remember, were a phenomenon really of the Principate) would therefore typically be returning to the same lives they had come from, substantially enriched by plunder.

That alone doesn't necessarily mean that discharged soldiers had no issues, but we know that inability to deal with agriculture was a significant question with respect to land settlement. When Cicero opposed the Rullan bill one of the arguments that he made to the people in a contio was that the urban population had no idea how to farm and would not be able to handle it. Gracchus needed not only to distribute land but to provide money for tools and equipment for the farmers.

Consider also the way Roman military service worked. Richardson points out that until Augustus as far as we can tell there really weren't any fixed terms of service. In the first century, BC soldiers would be disbanded typically after the end of a campaign. Caesar's troops were unusual in that they were under arms for a decade or more, but more typical were Pompey's veterans, whom he disbanded when he entered Italy. This was no longer true in the Principate, but for much of the army's history soldiers' service was effectively a break from agricultural work, albeit one that could last some time--we know from Livy that already in the second century, BC many soldiers were on campaign (especially in unpopular wars in Spain) for a decade or more continuously. I have a little difficulty talking about Roman soldiers "re-integrating" into "civilian life." For much of the army's history, that formulation doesn't really make sense. During the Principate soldiers were typically being settled on the frontiers as colonists doing exactly what they had done before, namely farming. They found themselves far from home, but were engaged in familiar activity, meaning that in many cases they were not so much "re-integrating" into civilian life, but creating a civilian life. Roman veteran colonies are extremely important to the spread of "Romanization," in part because veteran colonies typically established fresh societies in areas where there were no recognizable Roman societies in place. A more interesting question would be to look at colonies established alongside existing native populations, of which there were several.

As for the latter question, I dont think theres much evidence of unsuccessful settlement. There may be some in the Principate that I dont remember, but nothing comes to mind. Sallust says that Sulla's troops were unwilling to farm, but it's just as likely that many of the Sullan veterans simply did not have great plots, which as I recall is what de Ligt thinks. Sulla's veterans seem to indicate that there were many soldiers who succumbed to the same issues many farmers did--agricultural life was hard, and very uncertain. But we don't have a lot of direct evidence for that, and it would not be terribly unlike the lots of many farmers