There are two things that can make a donation problematic.
The most obvious is that it may be treated as a quid pro quo. You get the money, you must do something that the donor likes. People often argue that this doesn’t apply because things happened the other way around: the donor is supporting you because you did a thing that they liked. This still leads to questions though: would you have done the thing if it wasn’t a thing that made rich people or organisations want to donate?
More insidious is that it’s easy to grow dependent on donations from a small set of donors. This is the same thing that happens to a small company whose revenue all comes from one customer: you cannot do anything that would upset that customer and they have the ability to shut you down entirely by just not paying for a short time. Even if you’re taking donations from half a dozen big donors, if their interests are aligned then you effectively work for them if you have scaled your activities based on the expectation of those funds being available.
This applies to political parties and to F/OSS projects.
