Assuming such a state would be hostile to Israel, it would create a real dilemma for the US and its leaders. Such a state would've wielded enormous economic (as well as geopolitical) clout and used it more consistently in pursuit of a political agenda, notably pressuring other states to distance themselves from Israel. In light of the power of the pro-israel groups here, our politicians would have to walk a tightrope like the great wallenda or some such....
It should be noted that a removal of the State of Israel and a restoration of the Arab nature of the pre-1900 territory are among the goals of the late Mr. Abdul Nasser. So it's possible the OP, by its nature, precludes the existence of the Israeli state, or has it replaced by something more amenable to the super-UAR, like a puppet Levantine Republic or even assimilated into the Arab empire.
However, assuming a scenario Israel still exists somehow as an independent state, Israel's biggest game-changer is its NBC arsenal. Quite simply, the limitations that ruled Israel's adversaries before no longer apply. Egypt has massive population, but a relatively small front on which to engage Israel. Syria has a bigger population than Israel, but lacks the front size, experience, and training to take it on, and the Golan Heights are excellent defensive territory. Lebanon had limited its combat role against Israel up until 1982, mostly because it has the smallest population (even compared to Israel) and a very narrow front with difficult terrain. Jordan has the largest front, but barely has a population larger than Israel. Even working together, none of the "Frontline Nations" could have hoped to challenge Israel, mostly because cooperation would have been difficult between competing, mutually suspicious Arab Regimes.
A unified super-UAR, though, is a monster Israel can't hope to defeat. It has a single leadership, so cooperation is not an issue (assuming inter-service rivalries don't end up being game-breakers). It can shift units anywhere within itself in the name of "security", so we'd have Israel potentially having to face 90% of the attack coming across the massive Jordanian border, meaning it would have to spread its defenses all round and eventually get overwhelmed. If the Arab army is as weak qualitatively as always, it can just make up with quantity, effectively burying the Israelis in bodies. If Nasser somehow managed to actually make the armed forces somewhat efficient and effective... well, then Israel's absolutely fucked, isn't it?
And the USA or Europe trying to force the UAR to heel is going to end up eating a massive fuel embargo and loss of a gigantic market; the UAR has a market of over 150 million, whereas Israel barely has 5 million people. Quite simply, Christian Zionist sentiment aside, the US cannot afford to antagonize the Arab titan.