Now that they have released more monsters in the basic rules (for DMs), we can see the ghoul, wight, and mummy - although curiously not the ghast.
Ghoul
The ghoul is like a slightly-better CR 1 zombie, with 33 HP when you can first create them, 12 AC, and better attack and damage: +4 to hit for 12 damage (never changes). The real kicker for the ghoul is the (very low) DC 10 Con save or paralyze on a hit. It doesn't work on undead or elves (or any monster that is immune) and you can save each round to end it (otherwise it lasts 1 minute). Not that effective except en masse; with enough ghouls hitting you, you'll probably fail eventually - but probably not for long. For a 6th level spell you get 3 of these. You could have up to 25 of these at level 20, using all your level 6+ slots - although hordes of melee combatants are less scary than archers, and by level 20 you're comparing this to 40d6 damage over an entire battlefield, so... there's that.
How do they stack up? Well there's not a lot of higher-level monsters in the new Basic update, actually. One CR 9, one CR 10, and one CR 17 - that's it. (Even the Horde supplement only gives two 13s and a 16, and they are all dragons except the vampire - who is undead and therefore immune to paralyze). The CR 10 monster is a Stone Golem, and it's immune to paralyze and non-magical weapons, so it's a terrible example; they couldn't even hurt it (and neither could your skeletons). The CR 9 monster is the Fire Giant, but it can hurl rocks for 29 damage while the ghouls get close, and then do two attacks / round for 28 damage each, so they won't last long. It gets a +6 to Con saves, so it only has a 15% chance to get paralyzed (and then an 85% chance to overcome it at the end of its turns). So again: not really any kind of challenge. And the CR 13 Adult White dragon in the Horde supplement also gets +6 to Con saves and can shred these guys pretty well, too. You are probably better off with 10 archer skeletons for your spell slot instead of 3 ghouls (skeletons having 23 HP, 13 AC, and doing 10 damage with the same +4 to hit by this point).
Wight
A wight is like a CR 3 skeleton: it can use a sword or bow twice per turn, and once per turn it can life drain (instead of one attack with a sword) for 10 damage by the time you can make them; failing a DC 13 Con save your total HP is reduced by 10. This is pretty useless against anything you'd be fighting at 15th level, since even by CR 8 everything has around 150 HP - and healing affects PCs MUCH more than monsters. Technically you could do something with this, since up to 12 humanoids slain by this attack rise as zombies under the wight's control - effectively increasing the total number of zombies you could get going. But 12 zombies is definitely not as scary as 12 skeletons, since melee attacks are always limited by physical space.
Wights have 14 AC and a respectable 60 HP when you can first create them, plus they resist non-magical attacks (which most monsters will have by CR 15). They still have a poor +4 to hit and 11 damage on their attacks. You can make two of these at 15th level and technically three more at 17th level, but you'll never have more than five. They also have a pretty huge disadvantage in that they are sensitive to sunlight (disadvantage on attacks), which makes them much better as meat shields. Again: when it comes to two of these or 14 skeletons, there's not much reason to go wight unless you know you can bring along 24 zombies with them as well, and then it's just for the manpower; the skeletons will probably still do more damage.
Mummy
Mummies are like CR 3 zombies. 11 AC but a respectable 75 HP when you can create them, they also get resistance to non-magical attacks and a few more immunities (like paralyzed and frightened). They have a +5 attack that does 30 damage, which is nice, as well as potentially afflicting the target with mummy rot, but that's even more useless than the wight power against monsters (unless your plan is to hit them and run away), and it doesn't even raise additional troops. It can only attack 1/round but it can also on each round use a gaze attack with a VERY low DC 11 Wis vs. frightened (and if you super-fail, you are paralyzed), but it only lasts a turn and anyone who saves once is immune to any mummy's glare for a day.
There's really not much more to the mummies; they are meat shields at best, but their low AC won't keep them up long. They actually do the least damage of any of your options since you can never have more than two of these. If you'd rather spend your 9th level slot on skeletons - and you probably would - you can control 16.
We haven't seen ghasts yet, but they are considered on-par with the wights by the spell, and are probably less party-friendly, at least if previous editions are a guide. The main issue with Create Undead is that because Animate Undead has such a high upper bound on what you can do with a bunch of archers, unless you're trying to create a very specific effect, Animate Dead is typically better. If compared to only itself and evocations at the same spell level, the spell seems pretty balanced (or even underpowered) at all levels.