I have not yet added any sort of donations to my fiction, but it's something I've considered, so I'll lay out my thoughts here.
There's basically two things to consider:
1. What effect will it have on you?
2. What effect will it have on your audience?
I'll do the audience one first, because I think it's simpler to handle.
I don't think it has a huge effect.
This is just going from mostly my own perspective, but... I think most people will pay attention to the qualities inherent in the work first, and having a donation button or patreon will come second to that. For some people, it might add a sense of legitimacy; if you claim a release schedule, and have a patreon/donation button, some people might be more likely to read because they think you'll be more likely to stick with it.
On the other hand, unless your story hits really professional levels, there might also be a sense of disconnect; people thinking 'this author is overestimating himself', and shy away from it.
However, as long as it's not pay-gated, I don't think either of these effects are going to be too strong. I'm mostly just guessing here, but yeah. I don't think having/not having a donation/patreon button has a huge effect on how many people/what type of people will read your story, compared to the effect that having a better/worse story will have.
Now, effects on the author. This is the big one for me, personally.
On the upside, money! Having more money is nice. Knowing people enjoy your work enough to support you with their cash is probably really nice! Getting donations would definitely brighten my day, and having a monthly deadline or wordcount to hit on patreon might be really good motivation! I'd probably spend a fair amount of donations on beer and pizza, which could have a positive effect on my mood generally, too.
...Or, it might not. For me, I think if I started accepting donations or set up a patreon, it would raise the pressure of creating. It creates this implied social contract: since people donated, I'd feel obligated to post, and I'm pretty sure I'd be unable to fulfill that at some point, which could then make it harder to get 'back on track', which would start a vicious cycle that could crush all motivation to write straight out of me.
Sometimes, pressure is good, but as someone who has anxiety/depression issues occasionally, (nothing huge, but enough to cause problems with deadlines at times) I'm afraid that it could become an obstacle for me; I don't think I'm ready to deal with that yet emotionally. Childish? Probably. But that's part of who I am, at least for now.
Then there's the work involved... it's a one-time investment to get the account going, but if you make any amount of money, it also complicates taxes. Patreon especially might be annoying, since (and I'm not really sure how this works, never having done it) updating in two places could be a hassle too. This probably isn't a huge deal? But it's worth considering.
Especially if you set up a donation/patreon and no-one cares. Being ignored - especially after the work has been done - can be tough. Putting up channels for people to give you money and having them follow through shows concretely that people value your work, but putting up the channels and then having no-one donate also shows, concretely, that they don't. If that's not a problem, great! But the negative case is worth considering alongside the positive one. Take your audience size into account, maybe. If you have five readers, you're unlikely to get many donations no matter how good your story is. If you have a thousand, you're likely to get something, even if your story isn't the best. Is it worth your time yet? I have no idea what the tipping point is here... 100? 500? No idea.
Personally, I think my ideal method of monetizing my work would be to write a whole story beforehand, post chapters here while I revise/format the final draft, then at about 1/2 to 2/3rd's of the way through posting, put an ebook up for sale. I wouldn't stop posting, but people who wanted to support me or read ahead could buy that, and since I'd already finished writing, I wouldn't feel worried about crumbling halfway through.
...the problem with that, however, is it requires me to actually finish something. :p
I guess, in conclusion, I'd say that if your mentality can handle it, you have very little to lose except some time. But if you can't deal with it mentally, even getting money might not be worth it.
Anyways, just my thoughts. Hopefully they're useful to you. Take them with a grain of salt, since I've never actually done any of this.
- A dungeon-ish LitRPG starting with a shipwreck on a tropical island. Not isekai.