using powdercoat people who don't understand the needs of a bike frame will end in tears. But then so will using anybody who's unskilled to do a skilled job.
A good powdercoating will be a initial coat of rust killing primer, then a colour coat, then a clear coat. If they understand it won't be massively thick....yes thicker than paint, but harder too.
As for blowing through metal...that's choice of media and experience. The only items I've ever seen fail had already rusted through and were basically held together with paint (i have a lovely orange prestige windchime as a result). Water got in tje frame, collected at the junction of the down/ bb and ate it from the inside out. Seen the same on chainstays. You can hardly blame a powdercoater for a good looking, but ultimately rotten frame.
I always believed that the media might remove metal, so i took a prestige frame section (paint stripped) in and had it blasted with a frame power job....it measured exactly the same to within a thou'....thats 1/27 of even a prestige micro wafer tube. Im sure the same or more would come off wet sanding.
But i do agree that you need to put your frames into the hands of somebody who knows what they are doing and not a garden furniture blaster!