Is there any risk or drawbacks on editing SharePoint online list new form in SharePoint designer by Modifying the underlying HTML . I was trying to Hide some fields from new form only. I did it by removing or commenting the HTML on SharePoint designer. I'm just wondering if there is a risk associated with that because I'm changing the underlying form code. Is there a better way to do it without using two content type?
Yes, there is a risk. Updates from the SharePoint team will not show up in your form. The change you make is stored in the Content database and you have to update it manually. You can revert your change from the site settings page at a later time if you want, and then you will get updates from the SharePoint team.
Hide columns can be made just by setting the column to read only or hidden.
If that does not work out for you, a more complex but more flexible way is to create a JSLink configuration for that column.