1. It is quite harmful to one's privacy and becomes an impediment to BTC censorship resistance
2. It can leave one open to niche attacks and one becomes vulnerable to these cyber thieves who will extract private keys from signatures after a transaction has been made.
3. Quantum computers could extract private keys if these addresses are re used.
On the count of 1 as at above, talking about privacy, of course. The public keys were never to be private, that's why its called a public key. It was made to be opened to checks and verifications on transactions done on it but still, remains private as per the identity of whom is behind the address. Although, it puts it directly in the way of an attack to use an address way too often as the loads of coins on it makes it a target for hackers. This you can mitigate by simply generating new address as the need arises.
On the count of 2$3, there isn't a way for which, private keys or seed phrase would be extracted from the public address nor the transaction signatures as these are just a means to the authenticity of transactions done on the blocks and have no links to the keys of the addresses behind.
About quantum computers, of course they've got high processors and are relatively very smart but not up to the task of generating or guessing private keys just yet.