What uses would NOT be good choice for this reel? Im thinking about it for my horny toad/buzzbait setup 50lb braid.

It's a great reel for anything larger profile, faster geared reels are good for. The only applications I wouldn't slot it for are cranking/big swimbaits, and that's just because they don't have a slower gear ratio available. 

One would be fantastic for the setup you're slotting it for as long as you like to burn buzzbaits. 

When you look at these reels, they're really not much different than the 200 sized Curado/Scorpions. They're typically going to be used the same way, but people like them because they palm better, they're lighter, and some like them for finesse (especially the older 50's with a spool upgrade). They don't really fish differently, but they certainly feel different and can cut down on fatigue if you're on the water all day. 

A lot of serious Shimano fans love them, and I own a few of the 50's and a 70. However, and I may be in the minority saying this, but I don't necessarily prefer them over the larger reels with the same names. I have them for finesse applications and because.... well because they're cool and different.

It's personal preference. 

They'll be fine for what you're wanting to use them for. Wouldn't want them for anything you want a high line capacity for, like deep cranking or swimbaits, or a bait you want to fish with a slower retrieve. Other than that, you're good to go. 

I've used/still use a curado 50 for frog setups, the newer improved 70 with its cosmic "hagane" principles should make it better..