It’s exactly as risky as writing a cheque, in that respect. Basically it’s possible to be inconvenienced but not really out of pocket- fraudulent DDs are trivial to sort out via the direct debit guarantee, fraudulent withdrawals a bit more hassly but harder for peope to pull off and still, you’ll get your money back.
(the UK banking system is largely based around soft, economic levels of security, backed up with guarantees and customer protection. So rather than preventing all fraud, they just prevent most of it and pay you back for the rest)
The actual fraud you can do with direct debits and BACS payments is hilarious but not something that’ll involve some random personal bank account.