A friend of mine has had a negative experience insofar as they are working for a small company, using maybe only 15–20 certs and one day they started getting hounded by Let's Encrypt multiple times on the email address they used for ACME registration.
Let's Encrcypt were chasing donations and were promptly told where to stick it with their unsolicited communications. Let's Encrypt also did zero research about who they were targetting, i.e. trying to get a small company to shell out $50k as a "donation".
My friend was of the opinion is that if you're going to charge, then charge, but don't offer it for free and then go looking for payment via the backdoor.
In a business environment getting a donation approved is almost always an entirely different process, involving completely different people in the company, than getting a product or service purchase approved. Even more so if, like Let's Encrypt, you are turning up on the doorstep asking for $50k a pop.