Personally, I think consistency is a non issue.
Solgaleo GX is the only stage 2 deck with a non-Ultra Ball searchable middle evolution (Heavy Ball + Cosmoem). Plus, Heavy Ball also searches our current Solgaleo GX and the non-GX Solgaleo, making your offensive options a lot more stable. Heavy Ball is basically what Dive Ball was for Greninja decks, but Greninja is a "Stage 3" deck while Solgaleo is just a stage 2. Solgaleo GX is about as consistent as a Stage 2 deck can get in terms of evolution.
The Guzma + Espeon concern is a concern that every evolution deck has so I don't consider it to be a "flaw" exclusive to this deck. Also, Solgaleo GX is more Espeon proof than other decks with the global heal and max potion. I think Acerola is also a pretty good option if you are playing a 3 line of Cosmoem and set up your board appropriately to drop a Solgaleo on a Cosmoem. With the consistency that 3 Cosmoem + 2 to 3 Heavy Ball have to offer, you really are not going to struggle to evolve.
The worst part of Metagross GX/Solgaleo GX is that you need 2 Metagross GX and 1 Solgaleo GX set up to function as intended, and if you faced a fire deck that was never happening. With a pure Solgaleo GX build, you're only relying on one evolution chain (Brigette for 3 Nebbies), and you only need 1 of each GX set up to have your full system. If you're not facing fire, you can set up SuMo Solgaleo GX first and get an early huge OHKO, then the following turn set up New Solgaleo GX and get that energy loop going. It's a lot easier, a lot more consistent, and your energy loop doesn't get shut off by Garbotoxin like Metagross/Solgaleo did. If you're really good, you can Guzma up a Garbodor, OHKO it with the new Solgaleo GX while simultaneously charging 2 energy onto SuMo Solgaleo GX, then on your next turn, attach for turn, then Ultra Road and get a 230 OHKO on a GX.
I am currently working on a list to maximize consistency, even if I have to run excessive counts of some cards and then cut back later. This deck has so much power and defense that if you can simply guarantee a full Solgaleo set up by turn 3, it's basically GG. 230 base power to OHKO all GX's, and 120 base power to OHKO unprepared evolutions like opposing Cosmoems while accelerating energy onto your 230 base Solgaleo GX.
With 230 base power and 250 HP, Solgaleo GX is a battle of consistency.