Well, I believe that he should probably take out Ulic and Exar Kun with relative ease. Sure, they're both good, but they don't have anything to put them on a level with Luke.
Technically? Either, by virtue of existing evidence, are his superior by miles.
Exar Kun was declared Vodo Siosk Bass's greatest student in 600 years of training his Order in saber combat, possessed the technical ingenuity required in creating a form of combat for a weapon he'd never before heard of, and possesses an astronomically enormous advantage in the sense that the style died with him, and as such would be completely alien to Luke as a lightsaber combatant.
Ulic Qel Droma, when a decade out of practise with a lightsaber, physically past his prime, cut off from the Force (and as such lacking the many advantages that the Force provides (physical enhancements, greater reaction speed, precognition) and being forced to rely on his natural human senses when in the past he would have always relied on his ability to sense his surroundings with the Force), and holding back, was capable of stalemating an extremely formidable darkside driven Jedi in a saber duel, a Jedi so formidable as a combatant in fact, that previously she had been capable of slaughtering multiple Massassi Warriors (who were Force Sensitive alchemical creations that were directly stated to be capable of using the darkside with skill and precision and were created to combat Jedi and guard Sadow's secrets) with her bare hands in seconds. All of that, while having been forced to rely on one quality: his natural grasp of the lightsaber, and that alone.
For any non-Force Sensitive to be able to compete with a Force sensitive in direct combat is a monumental achievement, and even then, it would usually only be achieved by relying on factors such as technology, or the element of surprise, physical superiority, or even a natural trait they possess as being one of their species. Ulic had none of that; he wasn't even in physical shape or in practise with his weapon, and he wasn't fighting to kill either, and as established, Sylar wasn't just your average Jedi, she was an elite one. Quite frankly, the showing makes Ulic one of the most naturally gifted Lightsaber Users there is, and it can't even be put into words how ridiculously he outclasses Luke in the department.
They both do in fact; Luke hasn't ever given us real reason to assume that he was particularly skilled with a lightsaber, so based on what we know, he's outclassed.
Now that being said, I'm not going to deny that Luke's definitely superior to the two of these when it comes to force prowess, by no small margin in fact, and I'll be the first to claim that Force prowess makes miles more of a difference than technical ability when it comes to saber combat. However, Exar and Ulic aren't exactly slouches in that department either.
Ulic was already capable of blasting back two elite Jedi (his brother Cay and friend Tott) about thirty feat without any real difficulty whatsoever when not at his full level of power, and later, when he gains his Sith amulet, he skyrockets in power, to the point where he was described as rippling in darkside energy.
Exar Kun, as you know, was capable of absolutely decimating skilled darkside using Massassi Warriors with his amulet, was capable of freezing tens of thousands of beings in place, instantly, with absolute ease, and later, able to drain the Force from an entire race (in some hours).
Neither of them are that far below Luke in Force ability based on what we know, yet absolutely outclass him in technical ability.
With Ulic for instance, if he can perform like he did against someone like Sylar without even a connection to the Force as well as under all the conditions I listed, I doubt the superiority Luke possesses in Force ability will make that much of a difference against a powerful Force Sensitive, darkside driven, amulet wielding, physically in shape, Master Swordsman, going for the kill, in practise Ulic Qel Droma. And Exar of course was already capable of competing with Ulic before he even created his new, preferred style, which Luke would be completely unprepared against.
Now I'm not saying it won't be close, but either Ulic or Exar would take Luke in a lightsaber battle, and not a single person here has provided an argument to the contrary.