There are two downsides of using too many/low quality relays.

Paid relays are paid because they (ideally) put effort into speed/spam filtering/reliability. They can also aggregate from other relays on your behalf. Using a low-quality relay may result in a low-quality experience.

Secondly, notes are downloaded from every relay you're subscribed to. This is the good part of decentralization/censorship resistance but it can also mean using 7x (when using 7 relays vs 1) bandwidth or things loading much slower/out of order.

If someone who is not publishing to any relays you're subscribed to interacts with you, you may not [immediately] see that interaction.

One of the largest genpub obstacles here is that the pure Nostr experience requires effort and curation.