I want to say first that this is from what I've used it and my point of veiw, and I'm not the most amazing vgc player so...
Pros. As you mentioned, it has gmax resonance, as well as very good bulk. It can be used for something like perish trapping with dugtrio. This works suprisingly well, and lapras can hit the flying types who can still switch from arena trap hard by blizzard or ice beam, as well as thunderbolt if you decide to use it. Gmax resonance also puts aura veil, making it a bit harder to ko the both of them
Cons. Dynamaxing is only able to be used once, but it can ohko or 2hko many Pokemon while in this form. This makes your dynamaxer normally hyper offensive, and perish trapping doesn't even gaurentee 2 kos. It is a somehwat waste of a dynamax, as you'll get only 2 kos with luck, but if no then just 1. You can get 3 kos with a hyper offensive other dynamax Pokemon if you're lucky, and so it ends up being somewhat of a waste. And if the ko dugttio, the strat is over and your dynamax is wasted.
Conclusion: it seems good at first, but it comes to and end when you see how much other dynamaxers are doing in their 3 turns. Having a Pokemon with not amazing damage, as well as only setting rain and aura veil, instead of possibly leaving them eith one Pokemon, isn't so good. So, it wastes your dynamax for how little it can do, so it's not a very good choice.