The complexity of tax returns varies greatly. If you only have dividend income and/or bank interest then your tax return may be quite easy to complete; but where you have multiple sources of income, things can quickly become complex, and things can be missed.
In addition, HMRC will likely pay more attention to people with income over £200k per annum as they are classed as wealthy individuals by HMRC and are managed by a separate department.
If you fall into this category, it is sensible to ensure that your tax return is as robust as it can be to prevent unnecessary enquiries from HMRC.