That being said...
No, there's no inference or implication that any WordPress configuration will scale on WP Engine.
I would humbly suggest that your entire Web site is a refutation of this assertion. Even moving beyond the copy on the home page to the fine-print "Common Questions" section of http://wpengine.com/pricing/ , there's no disclaimer or other "are there WordPress configurations that WPEngine can't scale?" fine print.
My point here isn't that you shouldn't promise scaling, it's that it's not hard to see how a mismatch could arise between your expectations (scaling is hard, customer code can make it harder) and customers' expectations (scaling is easy, WPEngine has figured it out).
Jacques complained about lack of 24/7 support, but I shouldn't have tried to reply and explain the reason for that because 'there's no crying in business'. ?????
Yes. If a customer complains about a lack of 24/7 support, you say "I'm sorry, but we're not in a position to provide that level of support at this time." And nothing else. The reason why you're not in a position to provide it is irrelevant to the customer, and risks sidetracking the conversation into a blind alley about the reasons.
It also feels a little bit like pleading for understanding -- "cut us some slack, this stuff is hard!" -- which is off-putting. Of course it's hard, that's why people pay you to do it for them.
WP Engine should vet and screen it's customers so that they must prove their code can scale before we sell them an account? ?????
As I said, you already do some vetting by disabling plugins you know to be performance-killers. Look at this list: http://support.wpengine.com/disallowed-plugins/ , there's an entire category called "CPU/MySQL Thrashing Plugins."
I'm not suggesting that you need to give your customers a full CIA background check before bringing them on board, but you've already moved beyond just taking whatever code customers throw at you. If there's other things that need to be disallowed to ensure that WP sites work on your platform, the thing to do is to find out what those things are and add them to the blacklist.
Again, just my $0.02, IMHO, YMMV, IANAL, etc.
Use some common sense.
"infinite scaling" means _exactly_ that. If you don't want people to use accepted dictionary definitions of words like "infinite", you should not use them in your marketing.
A few telcos here in Australia have recently lost court cases about their use of the word "infinite" in their cellphone plans/marketing, and then cutting off or charging extra to customers who "used too much". The court, rightly in my opinion, said that the meaning of the word "infinite" is clear and well known, and any claim that it included "limited, of course, by common sense" is bogus. Be careful of letting your marketing team write too much hyperbole - occasionally you'll be held legally accountable for it.
I suspect if the amount in dispute had been an order of magnitude or two larger, Jacques may well have had a strong enough case to find a no-win/no-fee lawyer to go into bat for him... I know I'd prefer not to have to explain to a judge how those first two boxes in the first column on the WPEngine homepage didn't cover all of Jacques complaints.
I can create an infinitely scalable server and you can come in and drop a sledge hammer on it...does that mean I shouldn't use the word.
You have to understand the layperson won't know anything about tables etc and stating "infinitely scale" is a HUGE selling point for potential customers. If they wanted to be truthful they wouldn't say that, or they would say we CAN provide infinite scaling IF the following conditions are met.