With premium bonds, you'll lose a little with the bonds not being eligible for the draw until they've been in more than a month, eg if you buy in September, they won't go into the draw until 1st November.
Plus currently, the payout rate for PBs hasn't quite kept up with interest rates, which have increased a little.
And the payout is also down to luck. The closer you get to £50k, the more likely your prizes will be near the stated payout rate, which I think is about 1.4%. But as you're effectively gambling the interest you would receive on a savings account, you could get less, or more, possibly a lot more.
Over the last few years, I've had around £30k of PBs and my return has been just over 2% in 2019, 6% due to winning £1k during one month in 2020, 0.6% in 2021 and so far this year, I've won £200 on a holding of £25k, so overall I've probably had more than I would have had by putting the money in the best instant access savings account.