Client A requires VPN Client "X" that is exactly version 1.2.3.4.
Client B requires the same VPN Client "X", but force-upgrades it to "1.2.3.5" or whatever.
I now cannot connect to Client A without uninstalling the VPN client installed by Client B and reinstalling from scratch.
A gentle suggestion to the VPN vendors that write not-backwards-and-forward compatible clients like this: Go die in a fire. Or quit IT and go lay bricks or something. Actually don't do that, I wouldn't want to live in a building you build. It'll probably collapse if the wind isn't blowing at the exact right speed.
Similarly, Client C has a VPN client that is also the kitchen sink, and has a "profile" that overwrites half my system with its junk. It refuses to connect if I edit the profile. I have several coworkers who VPN in via Virtual Machines to work around this.
Many VPN vendors believe that VPN client downloads should require a support login. My feedback: Stop that. Just stop. Publish a straight up download link to every version. Please. Just do it. I promise you won't lose any sales because of this. Really. I promise.
More fun: A large gov org client of ours had spent nearly a decade hard-coding their "LAN subnet range" in a bazillion firewall and IP whitelisting rules in dozens of products. The VPN is in the Cloud and uses a different range (10.x.x.x/8), so now every firewall rule is broken for users connecting from home. 9 months into the pandemic, they're still working on it.
I have Gigabit fibre and a high performance computer. Downloads really do go at 800+ Mbps. I'm yet to see a VPN product that can crack 200 Mbps even during periods of low load and with a matching gigabit link at the other end. Most do 3-20 Mbps, and I've seen as low as 1 Mbps. This is entirely a software problem!
Microsoft deserves the IT equivalent of the Razzie awards for their implementation of IP-HTTPS in DirectAccess which is a) Slow as molasses, b) Limited to 8 cores on the server for no reason, c) Not fixed in 12 years, d) Still theoretically supported, e) Will never be fixed.
Vendors: Benchmark your products please. Once a decade maybe. Or two. No rush. It's not like there's a pandemic on or anything. Take your time.
I have to stop ranting now, I think my blood pressure is rising to dangerous levels...