I have a dedicated server with Liquid Web, and they migrated my ~30 sites from WHM/cPanel to IW a couple of years ago.
There are a few gotchas that you need to be aware of. Liquid Web does have a document regarding the migration, but there is at least one error due to out-of-date information. For example, it says that “InterWorx’s mail program does not support SMTP on port 465. If you use port 465, you will need to update your mail clients to connect on 587 instead.” That is not true; however, it is a good idea to upgrade your mail clients to port 587…
One question I would ask if I were you is what OS Liquid Web will be using for your new server. Since everyone is moving away from CentOS, it would be nice if they were using something else so that you don’t have to migrate again to get an new OS when that situation is settled.
One other issue I ran into was that I was using DNSSEC for all my domains, and Interworx did not support it at the time of my migration, so I had to disable it in WHM and at the registrar prior to the going live.
I was also using WHM’s backup to a separate drive, which IW didn’t support via the GUI; however, IW Support provided me with a very nice script I run via a cron as root that does the trick flawlessly.
Given your large number of sites, the other thing you’ll need to check into is SSL certificates. IW uses Let’s Encrypt for them, and if you’re bringing over a bunch of cPanel/Comodo SSL certs, you may run into Let’s Encrypt’s caps on new certificates when you try to set up a bunch.
Another issue I ran into was with ModSecurity rules in IW. I was running the OWASP rules without any issues in WHM/cPanel, but I had some significant growing pains using the OWASP rules with ModSecurity in IW with WordPress, WHMCS, and phpBB3. I currently have it working pretty well, though there are occasional issues with phpBB3 sites and WordPress that need to be addressed.
Overall, the migration was well-handled by Liquid Web, and I am very pleased with IW. I’m still using IW 6.x at this point, and looking forward to upgrading to 7.x once they have that working. You will be using 7.x for your migration, I assume, so you’ll have all the new bells and whistles.