Why is it so hard to loop on a freakin list of email addresses and send an email to them??
That's what we actually do! We have a queue of emails to send and we try to send each one of them with standard php function, if the host tells us the emails has been sent then we remove the email from the queue and we flag the email has being sent. Otherwise we try to send it again, 3 more times, and if it sill doesn't work then there is a problem with the email, so we just remove it from the queue.
Though sometimes depending on the host you are using there might be some sending daily limits in place which you are not aware of. And so what could happen is that the host will block the sending but will return to wysija that the email has been sent. This is something that is pretty difficult to control.
What you could do is get in touch with your host to learn about their daily limit, here is a post where we've started to summarise some of the most important hosts' limits : http://support.wysija.com/knowledgebase/lists-of-hosts-and-their-sending-limits/
Well, if you know that the WordPress service works bad, why the hell are you using it??? Is it so hard in PHP to count from 1 to 300 seconds?
Counting from 1 to 300 sounds like a fairly simple option :)
Unfortunately that is not as simple as that, there are so many parameters to take into account on all the different servers such as memory limit timeout limit, etc ... the best we came up with and I'm convinced there is nothing more reliable than that, is the queuing system we have.
Then the issue with WordPress' cron system is something else. We've started to work with it and are working on a separate solution to avoid those problems to our users. Though only a minority of WordPress sites see their WordPress cron system failing, this is something which happens sometimes when websites have for instances a trillion plugins installed doing tons of crons schedules at the same time. But when this happens, it simply result in keeping all the emails in the queue without sending it, so you are aware that something is going wrong and we're not pretending to send emails that we don't send. This is a completely different problem than the one you seemed to be having.
Hope this helps.