What's the hardest part of introducing new technology?

It depends on whether you're replacing an existing solution or bringing entirely new capabilities. When replacing existing you often have to overcome 3 major issue areas:
1. Some portion of the customer population won't want to change
2. Integrations between the old solution and other solutions (I.e., the service desk)
3. The risk of attempting to incorporate all the bad practice that exists with the old solution into the new one. 

Recognizing the above ahead of time can help avoid the worst impacts. 

Retiring the old one

OCM Organization Change Management, Agility to accept new, skill shortage for new tech.

The learning curve for older users and buy-in for employees.

In my experience, buy-in from the business is one of the hardest parts. The way to combat this is to include a majority of the stakeholders at the very beginning so that they feel part of the entire process. This has worked well so far, for me.