Poor Reinhardt...gone are the days of Elder God-tier...
Sorry, couldn't hear you over this shit still working for me.
What really shuts Dire Thunder down is Brave Ike, who nullifies 80% of the damage past the first attack AND nullifies hone. Brave Ike is probably the best all-around tank in the game. Brave Lyn is pretty fearsome against it, as well, and both of them are more commonly seen thanks to the "everyone gets a free brave hero" offer and generally being some of the best all-round units in the game.
And that's what happens when you mess with Sephiroth and Forever 17
Again, properly built, Reinhardt still stomps Sigurd. With an Atk boon (why wouldn't you have one?) and +10 (now a 3* summon, so easy to do), Reinhardt has a base attack of 48. If you have Hone Cavalry or Atk Tactic, plus Deadly Blow, that's another +12, and it's not hard to have an additional Goad Cav's +4 in the mix, plus as much as a +2 from ally and also another potential +2 from Summoner support, you can potentially get another +8 for a total of 68, or as much as 81 with triangle advantage. Sigurd has 17 Res (20 even with boon), so even with halved damage, that's 32 damage on the first shot of 41 HP, so 9 HP for the second shot. Even if we ignore the standard builds that use Quickened Pulse and Moonbow or Glimmer, the second shot can do 16 damage, so it's a kill. Even if you don't have every star align, and you're only putting out 60/72 with triangle advantage Atk, the first shot does 27 of 41 HP, leaving 14, and the second shot does 11 even before the special comes into play, so if you have Glimmer (people laughed when I mention Glimmer Reinhardt, but who's laughing now?!), then you're dealing 16 damage for the kill even with none of the other combat/support buffs. (Although you need at least some sort of buff if they have a Res boon, Distant Def seal, or fortified tile.)
The only thing that truly protects Sigurd is swapping out Close Def for double Distant Def... and I've seen that build ONCE in arena.
Also, new sword!Reinhardt insta-gibs Diadora. With DC, it doesn't even matter what phase, and with his natural Vantage (for the "preemptive double-counter" build), he's basically invulnerable against anything but blues and very high-def reds.
Basically, this is only a problem if you honestly expected to sweep everything that could possibly be thrown at you with one single character, which would just flat-out be a broken build. (Although the "God Grima" build comes pretty close so long as you activate Vantage safely...) At this point, it seems like IS wants the "weapons triangle" to be Dragons > anything not a Falchion > Falchion > Dragons.