No, they are not useless
You’re missing a paragraph right above that:
The arms are monk weapons and have a reach of 10 feet. The arms deal radiant or necrotic damage (your choice). When you attack with the arms, you can use your Wisdom modifier instead of your Strength or Dexterity modifier for the attack and damage rolls.
So you get extra reach, you can deal your choice of necrotic or radiant damage instead of bludgeoning and you can use wisdom for attack and damage. Now these aren’t all going to be useful in all circumstances, but for example radiant damage is good against zombies, reach can be useful if you’re dealing with multiple targets and are out of movement, and using wisdom for your attack is good if you prioritised wisdom over dexterity (and Way of the Astral Self wants you to prioritise Wisdom even more than other subclasses).
It is not a very strong feature compared to what other Monk subclasses get IMHO, but that’s true of the whole class in general aside from its 17th level features.
To address the comparison with Flurry of Blows and Martial Arts:
Martial Arts is always available. Flurry Of Blows is always available as long as you spend a Ki point, which you might be out of or conserving for another reason. The bonus action attacks you get from Arms Of The Astral Self require you to to spend 2 Ki points and a Bonus Action in a prior turn – you can not use them the turn that you summon them on (one of the primary weaknesses in this subclass actually), so realistically unless you face only one big encounter per day (not unusual, but not necessarily a situation the rules are optimised for) you’ll still sometimes use Martial Arts or Flurry just because they’re more convenient even if your astral arms are technically more powerful.
Keep in mind that even at level 11 once you can perform 2 extra attacks with your Astral Arms, you you still only break even in terms of total attacks done on your second turn vs. a monk that just uses Martial Arts every turn. Only at the third turn and up do you actually start dealing more damage. Vs. someone who just uses Flurry two times you’re actually still going to be behind in damage for the same amount of Ki points spent, and you can’t catch up in terms of damage to a Monk that Flurry’s every turn – you only get to deal comparable amounts of damage for less Ki points essentially. This often isn’t ideal because many smaller encounters barely last 3 turns.
As such, if your goal is to deal the maximum amount of damage in a useful way you should actually not use your Astral Arms at all unless you can prepare them before the fight starts. The Astral arms are mostly useful for their non-damage benefits, and if you're not going to be making use of those you’re better off without them until level 17.