I don't feel I've had enough experience of offline to comment on his tier placement, just the amount I got to play Grundy's bad matches offline at final round made me think they weren't as bad I first thought. That said, they are still pretty damn bad.
To me the biggest downside to maining Grundy is that some of his worst match ups are against some of the most common characters in the game, at final round I played Aquaman, Batman and Zod in tournament. That's 3 straight matches that are incredibly tough and those characters are quite common. It's not that bad when you're playing online when things don't matter, but relying on Grundy 100% for tournament is one hell of an uphill battle. Even if I had beat all three of them, there's still a pretty damn high chance I'd come across them again.
One thing that I haven't seen so far in this thread and as such curious to get other Grundy players opinion on is how entirely dependent on meter I feel he is. It's a huge problem for him IMO, his meterless damage without a BGB isn't great and his raw meterless damage is shit compared to what you get for 1 bar. Grundy needs meter to do anything really good and as a result once the red bar comes around you can be a in shitty situation. I hate having to use 2 bars just keep the life even when they could give me the match, I hate having to use 1 bar to keep 15% on my life when it can be used to take 60% of the opponents.
I think Grundy is still a great character, but he is not without his faults. The first thing people take notice of is the damage, which is awesome no doubt. But damage isn't everything, Cyber Zub zero could do 40% for no meter, ermac could break 50% with no meter but at the end of the day high damage alone is not enough.