A few off the top of my head: juggling packets between userspace and kernel resulting in poor performance, tacked-on hacky features (hmac authentication), fragmented and often outdated documentation (the community wiki is a mess), homebrew key generation helper tools with hardcoded openssl versions, no native support on Windows/MacOS, autostart services on Windows are a mess (OpenVPNService, OpenVPNServiceInteractive, OpenVPNServiceLegacy), faffing about with socket buffer sizes on windows to get any sort of throughput and so on...
I don't know where you got your information...but you can achieve wire speed with OpenVPN over a 1gb link. If I had a bigger pipe, then I could run a bigger test, and expect the same results. The rest of your points fall under "it's not easy for average users" - which I agree with.
That depends on how much horsepower you have on both ends. I run my vpn off of a cheap cloud vps, which does the job, but doesn't have a lot of cpu power. I probably tripled my speed from openvpn to wireguard, all though both fail to max out my internet connection.
This. I'm using OpenVPN for ethernet bridging, which Wireguard cannot do.
Tinc would be a nice choice for this purpose.
I've done bridging over wireguard. You just set up a gretap tunnel to run over the wireguard tunnel.