Coin games on mobile are fine, but if you play for long enough they lose all point other than friendly contest against other people. You get to a balance of millions of coins, in the top Diamond level or whatever, and there's nothing to play for other than maybe getting a different coloured shirt. They're a good way to play a quick one-hole game to maintain a "consecutive days played" XP bonus though. But don't spend any real world money on coins, ever. Ultimately there's nothing worth it that you can buy with coins, unfortunately.
In terms of hitting an equipment wall.... well yes, that happens. I've been outgunned by competitors in head to head for months because I'm using cheap balls that add no distance and an older driver, but still able to compete because I was doing well on the approach and on the green.
Now that I have bumped up to Legend tier and further back tees, my gear is woefully non-competitive. Par 4s are out of range, long par 5s are seeing 150-170 yard approaches. There's a few holes where I can't REACH the fairway with my drive, it's that tragic. My L48 driver and 64 cent balls aren't getting the job done, no matter what I do. So I'm grinding away on surveys to get the credits to upgrade my sticks, and will dabble in some slightly more expensive balls (still not leaping to this $6-9 per sleeve stuff though).