I'll confess that my reaction to the Thunder was (and in many ways is still) the same as that young lady from Victoria: this is a good mech, sure, but not as good as it could or should be.
The key problem with the AC20 models (leaving the artillery aside) is quite simply a nearly total lack of ranged weapons. If it were a Blitzkrieg or Legionnaire or something at 7/11+ (even 6/9 like the Stormcrow B would be better, though its medium lasers out range the Thunder by 60%) I could accept it's a mech to race in and destroy. In the Succession Wars maybe 5/8 would have been enough (granted, the Andurian wars are the last experience the CC has with combat, so they haven't learned about real true modern speed and power from the Clans).
Now, sure, once you get to two hexes out the Thunder is a monster. It's got as much armor as it can, and an AC20 and three MPLs are no laughing matter (unless you're the one firing them). With three tons of ammo, you can definitely take some shots you might pass on in other mechs, and in the more modern era gives you some options as to special ammo. Yeah, taking precision ammo just kills you on an AC20, but the idea of having precision ammo and pulse lasers and taking a shot at some fool in a Spider or Ice Ferret who thinks to zoom by is just delightful.
I do like the stealth upgrade, since it makes closing across an open field a lot easier, and puts a bit of decision making pressure on your opponent: take a shot at the scary AC20 mech that's moving pretty fast and has stealth armor at medium range, or its supporting mechs? Or if it's supported by stealth Archers and Pillagers and so on, then ouch. Yeah, it would have been nicer if it had a more Ti Tsang like bevy of small lasers to fine tune the TSM (a 70 AC20 mech closing at 7/11 is, again, nothing to laugh at unless you're doing it), but the idea that it's more a get out of jail free card for when you have to leave the stealth on and fire all your guns has some credence.
As for the Arrow variants, I don't need that fancy an artillery platform. The speed is nice, sure, so you can keep up, but you've already got Arrow IV Regulators, which are faster in open terrain, and the jumping Catapults are better in broken terrain. They're not bad, good heavens, but they don't need to not be bad. They could afford to be bad. And yet they're really good. Why?
Lastly the slow one with the jets? I love jumping, and while I think often IJJs are miss applied victims of new toy syndrome, I like them here. Big giant guns shine in broken terrain, where you don't have the open lines to make a seven hex run, and IJJs shine in that same terrain. Let your infantry or Ravens or other scouts find a target, execute a six hex jump to get nearer to the fool, and unleash the beast. I'd have just stuck to the AC20 here, rather than the Thunderbolt, for reasons already enumerated. You need a good pilot to pull that off, mind, but if I'm being Canopan I'm probably giving these mechs to better pilots anyway. As is with the T Bolt I don't know its better, but it is an interesting direction to go and I think it could be refined to be better.
In conclusion, I think it fails as a heavy cavalry mech by mainly being a faster bigger Hunchback. This isn't a bad thing, but it makes the mech a specialist, and not a generalist like the Falconer or Jinggau or Hercules or the Clan heavies that inspired the genre that can roam around the field dealing death. The Thunder has to be used with care, held back and unleashed at the right target at the right moment. It can absolutely excel in that role, sure. But it is a more limited role than I think the mech was made out to fill or than it could or should have filled.