I disagree. It makes the assassin reactively dangerous when they, perhaps more than any other class, should be proactively dangerous. An assassin, a "blade in the dark", is at their strongest when you have no idea they're there. If you're facing them and swinging a sword at them, that is... exactly the opposite of that. When that regrettable situation occurs, it's not time for the assassin to strike, it's time for them to back off and think about a new plan. So something like "move half your speed without provoking opportunity attacks" would be a fitting reaction. But a riposte? That's for battlemasters and berserkers who stride boldly into the melee and dare you to come at them bro.
Eh, I'm done with this particular argument. The entire notion that assassins should be made to run away and find shadows to strike from is bunk, IMO. Assassins should be trained to fight, because no plan is so perfect that it cannot fail, and fights
will happen, even to the absolute apex paragon of assassination. Period. You don't fail to kill your target simply because the vaguries of luck and timing caused them to see you before you were able to strike the killing blow from hiding. You train in efficient killing well enough that their every move against you creates opportunities for you to kill them.
And "reactive" deadliness isn't counter to "proactive" deadliness, even in balance terms. What matters in balance terms is deadliness over the course of a round, combat, and day.
Watch John Wick, or Jason Bourne, or any other assassin movie. When an enemy or mark tries to attack them, they counter and the enemy or mark dies. Often, the counter and the killing stroke are basically the same motion.
There is no such thing as "not getting in fights". The assassin is deadly in or out of a fight. THey're deadly when put on the defensive, and when allowed the offensive.
Assassins should be even deadlier when they can watch and wait and strike at the perfect moment, which is what Shrouds do. They should
also be deadly the rest of the time.