Looking at the reasons for failure of other leagues as well as their successes, I don't think the UFL has much chance
There were many cities of substantial size without a pro football team when the AFL and AAFC started. Today the only unreresented municipalities of size are LA, San Antonio, perhaps Las Vegas and then what? Salt Lake perhaps and then it gets really hard to identify other cities. Hartford? Birmingham?? Even a small league of 8 is hard identifyas franchise sites. By comparison the AFL could put clubs with no competition into major cities or geographical regions like Boston, Miami, Denver, Cincinnati, Houston, Seattle, KC/St Louis , Minneapolis and San Diego even as they were being driven out of LA and Dallas and Minneapolis by the NFL. There were still large metropolitan areas like Atlanta available to transfer a team.
Virtually every viable market save two or three, already have Teams.
The second constant failure was the greed of the owners. The AFL and AAFC established itself for a few years before a player war developed from a low base. That low base doesn't exist today. Perhaps if the league set itself up as a single company with divisions (franchises) and signed players to the league and then dispersed them every year in a draft to ensure competition and control salaries, they might survive, but how do you build fan loyalty or continuity? The Browns were simply too good for the AAFC (and the NFL), but it killed the AAFC.
The third problem is simply over exposure. Fan interest is cyclical. Baseball and Boxing were fan favorites at one time, as was the NBA for a short time. Just how much football can you broadcast? I suspect that this may be the highpoint of TV money. I'd wager a few bob that the next TV package will pay LESS money to the NFL than the current one.
This does open a opportunity as players discover that the cap is LOWER. Few can be resigned for even the same money. Many would be asked to take pay cuts. Many players will be disgruntled, but where is the resources for the UFL to compete and sign them? That time is still several years off when the just renegotiated TV deals expire.