Technical issues with Heroes III.

Guys, when you ping someone in Hamachi, is everything always okay?

No. Sometimes there's a ping, but you can't connect. And sometimes, on the contrary, there's no ping, but they're playing normally. When your computer pings another, it uses ports, some of which are open on the pinged computer, and some are not (which the game might use).

Ping ~1000, packet loss up to 25% - is this a sign of... unstable gameplay?

This is a sign of a bad connection; the game will likely be unplayable, especially on large maps.

Added 2 minutes later
But I've already said, I don't know which ports people use, so it's worth first disabling any services that might be blocking the computer's ports, and then you'll see if the problem is with the connection.