Pretty much any text can be used for an account name as long as each account gets a name that is unique. Valid email addresses will meet the criteria for unique file names if you include the full address.
There are only two drawbacks and both can be handled fairly well in most cases:
1) people change their email addresses, so you will need to decide if you want a way to manage the change in account name to accomodate a new eamil address. If you have multiple files or you use an auto-entered account name in a field in your table, this change may require a bit of scripting to find and update all instances where the account name should be changed.
2) that pesky @ sign can be an issue when performing finds. @ is a find operator so when you enter an email address as criteria in a find request, you have to enclose the email address in quotes to get Filemaker to treat @ as that character instead of a 'wildcard'.
It's usually the Password, not the account name that's a challenge for users, do they use a password that they use in other systems or try to remember a brand new one. The person's own name can often be used for their account name and that might be simpler even than the email address though the chance of duplicate names will then need some special handling.
Thanks for the warnings.