Is there any particular reason why having many subdomains and/or ftp accounts / addon domains / domain pointers, sql databases, etc. on CPanel is bad?

If I give my clients 100mb space for example, there is distinctly finite amount of emails and sql databases the client may have regardless of the 'unlimited features'.

But, when I give the client 2GB and he creates 2000 subdomains and 1000 ftp accounts, will I run into problems and of what kind? (I mean, if Icharge him accordingly , say $20-30 per GB storage and 10-20Gb transfer)?