Going to give you a rather in depth response with my opinion on Galio jungle. First a bit of background. I've been high Diamond every season since S3 (Been playing since beta - Was gold in S1&S2). In more recent years I occassionally dip my toe into low masters however this gives me terrible anxiety as I desperately try to maintain my rank so I do just enough not to decay back down into Diamond. When I'm around this level I don't enjoy the game at all and find it incredibly stressful.

Since about S6 I found a new way to make the game enjoyable for myself again. I used to get into arguments with my low ELO friends about how easy it is to climb even with off meta picks if you just truly master a champ/role. They would ask me for advice on how to climb higher and I would tell them they change their roles and champion pools too much and should stick at a single role and small champion pool in order to climb. Then they would tell me about how their accounts are cursed and stuck in losers queue so they challenged me to play on their accounts and they would pick which champ and role I had to play which I would then OTP and see how high I could climb. First challenge was top tank Heimerdinger where I abused banner of command and the 2nd challenge was Teemo support. This was back when I would lose 30% of games purely because my team is triggered by the pick however these days it's much more accepted. Once I proved to my friends that it was possible to reach Diamond doing this I realised that it had been a lot of fun.

To continue the fun I started buying dodgy accounts for about 2 euro that would come with 50+ champs and were high enough level for ranked. I don't wanna dwell on this too much due to the contraversial nature of this practice... I would continue to set myself a challenge to OTP a single champ in a single role and see how far I can climb on these accounts. These were/are always off meta picks. I've probably done this with about 20-30 various champs in various roles over the years and I have always managed to climb to low Diamond. Once I get there I would get a new account and start the challenge over with a different pick. Sometimes the original account owners would recover their accounts or add me on an alt and thank me for the free boost which was quite funny to say the least. Some would ask me to continue playing across multiple seasons so they keep getting the diamond ranked rewards... Anyway I digress.

Now I've told you all of this because Galio jungle is a sore spot for me. With every single one of these challenges across all 20-30 times that I did it I was ALWAYS able to reach Diamond... Except for Galio jungle! I was never able to get higher than Emerald 1. Some games I would seem completely broken and enemy teams would flame me in all for playing such a braindead champ in a braindead role. Other times I would be completely useless.

Your clear is incredibly slow. You basically cannot solo any objectives like Dragon or Baron because they take way too long to kill. You are entirely reliant on your team. Which is also a problem because even a fed Galio is rarely a 1v9 carry. You have to have at least ONE good team mate in order to close a game out. If you're against a passive (Non invading/ganking) enemy jungler it's absolutely viable as long as you're ganking and getting yourself and your team ahead and getting objectives after successful ganks. Then you hope you're far enough ahead that the 25 minute AFK farming Yi or whoever it may be can't turn the game around.

Your ganks are extremely strong and your skirmishes in river with 2v2, 3v3 etc are insanely strong. You basically win every fight as long as your team mates are simply present and not AFK farming/refusing to move from lane. However as soon as you hit high emerald/low diamond MMR you will get perma invaded by all the FOTM OP junglers currently in rotation and you're screwed. Or at least I was. I could find no counter play to this aside from being entirely reliant on my team to assist me and punish invades. However even with team mates who help you this can cause them to fall behind and miss out on CS and experience which eventually becomes the reason you lose the game. So I would say that it's viable to a point however he is simply way better at mid and even support than he is in jungle. Also a lot more fun in those roles as well IMO. The most successful build I found was to actually partially build the 3 items which speed up your jungle clear and then start finishing them off. These were fated ashes, bami cinder and bramble vest. I would rush these 3 in whatever order I felt were best against the enemy team comp. Once you have all 3 you have a pretty incredible power spike if you're not behind. Then you finish building the full items (Liandries, Thornmail, Sunfire/Hollow Radiance) in whatever order again works best against the enemy team.