I've deployed VoIP for our own company, several times, and as a long-time business telecom/VoIP broker, I've deployed it for clients, too many times to count.
The hardest part is accounting for all of the little details, pre-install... mid-install... and post-install. Such as, knowing every single phone number, every call group config., every desktop's software integration, every fax ATA, every phone hardware config, who needs a headset, signing all the paperwork correctly, coordinating multiple people's schedules for the installation, etc.). The key is to be very organized up-front, which makes managing the details during the install, much easier.
I've also seen a lot of IT Departments who are (to no fault of theirs), looking at providers that are charging $20+/user, and assuming hosted is too expensive. Providers who are charging that amount are targeting tiny companies. Large companies can find much better alternatives if they know where to look.
Sales reps... I don't know many industries without their share of bad salespeople.
So what can you do?... Well, if you are about to jump into a phone system project, here is some good content that will help you prepare:
How to Shop Hosted VoIP Like a Boss [Spiceworks How-To]
Which Phone Technology is Best for Our Company? [step-by-step Infographic]
SD-WAN will fix business VoIP [Network World]