I think there are three main issues with religion in Civ 6:
- Passive spread is too weak
- Follower beliefs are unbalanced
- There are minimal benefits to spreading religion if not going for a religious victory
The first is straightforward - the passive spread of religious influence is weak, which leads to the missionary/apostle spam. There's no real ability to just let your religion spread organically.
For the second, the follower beliefs can be very strong. But the AI always takes two of the strongest first - Choral Music and Feed the World - which means if you want one of those as your follower belief on higher levels, you have to really tank the early game to get there (or maybe play as Russia). Looking at the remaining beliefs, Work Ethic obviously can be strong if you have a couple of good locations (lots of mountains/natural wonders) or a terrain pantheon. If not going for Work Ethic though, the remaining ones mostly just have niche value - relic-heavy cultural victory with Reliquaries, Divine Inspiration with Qin. Jesuit Education is not bad but not as good as others. So what this means is outside of those niche cases or a Work Ethic game, you're likely in a better position if you get one or more cities converted to Choral Music/Feed the World and then spread that religion yourself to your own cities, which is counterintuitive. The reason that's the better play is related to the third issue...
There are benefits to spreading other than religious victory, but they are comparatively light and pale in comparison to having a strong follower belief. For founder beliefs, something like Cross-Cultural Dialogue or World Church can provide a passable amount of science or culture, but you're honestly probably better off using the faith to buy a settler, found a new city, and build a campus/theatre square. None of the worship buildings are terribly useful unless you're Arabia (can build for free) or are going for a religious victory. A little extra science, extra amenity, or extra diplo favor isn't bad, but is more readily found from other sources. Finally, the only enhancer beliefs that have much benefit beyond aiding in spreading your religion are Crusade/Defender of the Faith.
The takeaway is that in the vast majority of my games - i.e. not playing as Russia, going for a religious victory, or with a specific niche strategy - the options are (1) use work ethic, (2) ignore religious play entirely, (3) try to suck in someone else's religion to get a good belief. I'll still end up building holy sites to generate faith, since faith is so useful in other contexts, but I think the game would be better with stronger benefits from founding and spreading a religion, short of full religious victory.