I believe that there are no downsides to allowing savefile editing. All it really does is just cut down the time until the inevitable. Eventually someone will get that one seed with a god item combo even on the full savefile, only difference is that it'll take longer to get such a seed and that it'll take even longer to beat a run with that kind of seed.

Removing (at least a certain part of) undesired items just allows for more (not perfect) consistency and puts the perspective more onto the player input instead of just rolling a god set of numbers. It's not like the whole 'roguelike' part is completely gone, just not as frustratingly impactful as it is currently at times.

I would like to add more but I feel like I would just be repeating people above in the thread.

I have asked about what the reasoning was behind using an 100% save file in the Gauntlet mod, and the answer I got was because it was the easiest way to make sure that everyone in a race had the same item drops during a run. Some players seem to think that it exists because of other reasons that honestly just sound like their own personal preference when playing casually.

Getting rid of some useless unlockable items from the pool does not make the game any easier as some may suggest, it makes the runs more consistent.

There is an argument that having the 100% save file makes you deal with everything the game has to throw at you, which is partially true but this only matters during races since you can't just reset to a new seed. This is a speedrun which means that if game doesn't throw the right things at you enough then you are looking at a reset. No one is going to bother continuing to fight a Xi Construct on stage 3 with no items because they have a race they need to finish.