Frost can't go afk in any content where you actually need him rather than use him to be extra lazy. Gotta be casting globe every four seconds in high level content because even the absorbed health from level 100+ enemies won't keep it up long.
As for Gara, she became strictly inferior to Frost as a defensive frame when Mass Vitrify got nerfed. Remember, it used to be invulnerable just like Snow Globe always has been, except it was invulnerable for the full (long) duration rather than the first four seconds. That on top of it blocking enemies entirely rather than just blocking fire from outside and slowing (+freezing with the augment) ones that walked inside. She was the definition of power creep and got nerfed for a reason.
She's in a decent spot right now. 90% DR is right up there with the other frames that are survivable but not outright invincible--still strictly inferior to Rhino, Limbo, Inaros, Frost (on defense), Wukong, (effectively) Oberon, &c., but about as good as post-nerf Trinity, Mesa, Nova, &c. and she has mid-tier area defense and single-target ability damage.
The Splinter Storm -> Mass Vitrify -> Shattered Lash damage buff is... eh. Yeah, technically and hypothetically infinite scaling, but it's very short ranged and takes a shitload of casts of three separate abilities to get damage numbers that you can get from a single cast on a number of other frames. And, functionally speaking, it's not really infinite scaling in a practical sense because you're not going to have the time or energy to recast the full sequence enough to deal with high level enemies. High level content is all about CC abilities in conjunction with weapons precisely because most damage abilities just can't be used efficiently against health pools measured in millions (much less against armor if you don't have 4x corrosive projection).
Bear in mind that the nuke from hitting the wall with the lash doesn't scale well. It's only good for killing low level trash, and there are a pile of better nukes out there. The explosion range doesn't get very big even if you max range at the expense of strength.
Before the nerf she was absolutely busted, even worse than Limbo for making defense objectives trivial.