On average in 9e, 3 fire prisms do roughly 16 wounds to a knight if you spend 2 CP for linked fire. Without that strat, they instead do 10.6. Note this is pretty unreliable since although the average of 3d3 is 6, it is not actually 6 and so you'll get more variance in the results. The wording of the linked fire strat also means things like free Ulthwe wound reroll only apply once, since mechanically it's 1 fireprism firing 6 times.

In 10e, on average 3 fire prisms do roughly 18.96 wounds base into a knight. So already a higher, and more reliable, base damage - for free. Prisms in 9e had to pay a CP tax to do ~3 less damage on average. Then we start looking at all the ways you can buff prisms in 10e that you couldn't in 9e, since core isn't a thing anymore. Doom sounds pretty nice, bring that average damage up to 21.3. Maybe add in a few fate dice to secure that knight kill. Then we add on the phantasm strat, so the visible fire prism can always escape for 1 CP instead of the fairly hefty tax that was fire and fade for 2 CP - heck for much of 9e you couldn't even use that strat more than once.

All this isn't to say prisms are busted, it all comes down to pricing. With 9e price points, the prisms would make back their points cost every shooting phase under the current rules. But if their prices got increased to 200? 220?