I think the welded loops are better now than they have been in the past. Just a few years ago many of the welded loops could be pulled apart with two pinky fingers. Sandy Moret was cutting the loops off brand new sharkskins in his shop and whip finishing new ones prior to selling them.
Doubled fly line also is bigger in diameter than a braided loop and harder to get through the guides, but I would use an Albright before a nail knot if choosing between the two for a big fish rig.
On the fly lines I have loops on, I make my own from Gudebrod braided mono and make them much longer than the bought ones which can fall apart because they are not made properly (nor are they long enough in my opinion, for big fish).
Also the loop on a properly made one can be slid open wider in order remove a backing or leader loop and can be made biggger in the first place to facilitate changing lines, leaders etc.
I use two nail knots of 8 or 10 lb mono or fluoro to hold the end of the braided loop onto the fly line and then fair them in with UV cure flexible glue. How to make them below:
On my large weight rods, I've eliminated loops on the leader end and stuff my leader butt up inside braid so I have no knots there either except for the small nail knots of 8-10 lb test. I normally use longer than normal leaders and the line/leader connection must travel through the guides very smoothly.
Here is a 10 second video showing how even a little fish can create havoc at the end of a fight. In this video my buddy, the very stocky guy, was in control and scared the fish trying to swim under the boat by whacking the gunwhale with his pipe, which he keeps in his top pocket. It happens in the first 4 seconds of the video which is only 15 seconds or so long. Had he not done that, the angler on the bow would have been in panic mode, and could very easily have busted a tip.
When using braided loops, here is how I attach them to get a minimal diameter.