The -20 really shines, 40 points of damage for 15-tons, that's better then LRM's get
The big flaws with the Ultras as a whole is that they are lumped together as a single weapon system, when the Ultra 2/5s and the Ultra 10/20s are vastly different weapons. If the Ultra 2 and Ultra 5 automatically hit with 2 shots every time they fired *and* had zero chance of ever jamming, they'd *still* only be sort of good. The Ultra 10 and Ultra 20, if they hit twice, are kind of devastating (especially the Ultra 20 which can kill a mech outright in a single double hit shot in a lot of instances), and the jam chance is a reasonable risk most of the time, especially given how often you fire the big ones relative to the small ones:
-An Ultra 20 with 4 tons of ammo (20 rounds), if you fire on double every time, isn't particularly likely to jam (1/36 of 10 shots).
-An Ultra 10 with 4 tons of ammo (40 rounds) gets closer to jamming, but still isn't certain with 20 shots.
-An Ultra 5 with 4 tons of ammo (80 rounds) will likely jam on 40 shots.
-An Ultra 2 with 4 tons of ammo (180 rounds?) will jam multiple times on 90 shots.
I mean, yeah, these numbers aren't particularly realistic for the 2 and the 5 (in terms of total ammo and number of shots), but where the big guns that are worth the risk of jamming (due to the strong payout if you hit twice), they also fire a lot less often due to shorter ranges and less need to fire them often--you fire an Ultra 20 on double shot 3 or 4 times? You are probably done firing. You fire an Ultra 10 on double multiple times? The game is probably done before the thing jams. The Ultra 2 and 5 need to fire a lot to be worth having, and the more you fire them, the more likely they are to break down. And given that their advantage is range and deep ammo bins, they are going to see them fire a lot at longer ranges in an attempt to maximize their use. So the ones that least need the break down rule to balance them out are the most likely to break down and the big ones, where the break down rule makes sense, balance wise, are a lot less likely to break down.