Over the years we've seen them certainly but I'm not as intimate as I used to be with the email reporting currently.
Here are random thoughts when I've tried to troubleshoot...
Trying to nail that down a cause is full of variables both with CVENT, the interwebs, your ISP/email server, and your internal network.
Pick your battles and report ones that you see as sever and have a lot of examples to put in a ticket. Don't call, submit a ticket yourself. Cause its complex, don't expect support to respond quickly unless you aren't able to send anything (in my experience) . The responses generally aren't satisfying but if you have enough tickets to point at the frequency you'll have more data to find patterns and you'll get more examples your account reps can back you up with to find CVENT help.
You should be able to verify the time signature of when Cvent fires them in the reporting, including when the first and last went out to determine how long it took to send. If you have an individual email with the header information, you can also compare their email history in cvent to compare differences. "emails delays again... same event.. same campaign.. same time... " etc. Sometimes its the time of day of your send; we fools around with off times to get off peak traffic times frequently. Also break up your sends group with custom tags filters into different times of day to see if there's a better time to send?Since you're sending to mostly internal sources, be sure to verify its not internal networking failures, ISP provider, firewalls, spam blockers.. etc. first cause you have the most control.