After 4 beers, probably pretty good.
Better save the beers for later.
Well ... seriously ... I would have to say the chances are obviously "not zero" but after having done about a dozen now, it really does seem basically as straightforward as using a hole saw on plywood. The only difference is that you go a LOT slower and a lot less pressure on the drill. These are basically just like a typical hole saw bit for your drill, only difference is they're diamond-tipped so you almost just "sand" the hole out of the glass.
With 20g tanks, the glass is not very thick, so it won't take more than 2-3 minutes per hole. Again the key seems to be to take it slow, and really back off on the pressure just before you punch through so as not to have the hole come ripping out of there on the backside.
It sounds worse than it is, really.
PS. You know, one thing about multiple tanks chained together is it will be like having baffles. It should really cut down on the odds of microbubbles (hopefully).