(I'm talking about VPN > Tor case, not Tor > VPN)
Why do so many people say it's useless?
I'm very sorry, but all that information is outdated and irrelevant. DPI can't detect TOR inside a tunnel. Please read about VPNs and how they encrypt traffic. You can also use VLess/Vmess/Reality to completely fool DPI. Even if the VPN provider keeps logs, it will give glowies more work. They will have to contact the VPN provider, etc.
Or am I missing something? Are there any REAL downsides?

upd. Also about the latest update - WebTunnel bridges. They don't fake TLS fingerprint to mimic browsers (Chrome, Firefox, etc.), so DPI can be easily trained to block them. You can check it yourself. Making them useless