Here is the issue. On many Weber grills they use distinct valve spools based on the intended fuel. They way the valves work is this. The main metering orifice does basically nothing at less than "full throttle". All the metering for anything from low until you get to full is done by the spool itself. All the main orifice does is limit max flow. You could almost run it without the main orifice if you only did it on low (please do not try this it's for illustration ONLY).
Where the issue comes in with the shysters on FleaBay selling "conversion kits" is the sell you main metering orifices ONLY. So if you have an LP grill and you install NG orifices and send it the proper flow and pressure it will operate just fine. At high ONLY. When you start cutting back to medium or low, you can get flame outs because not enough gas is flowing to keep the burners lit. Flameouts can be and IMO are VERY dangerous. Because you have no flame but gas is still flowing and collecting under the hood. Now if you have say one burner go out but more than one are lit, you can get a pretty good fireball in there. Will it blow your grill off the deck? No it will not. But, it CAN make a pretty good "woomph" if you should open the lid in that condition. Or it could build up and blow the lid open.
Now take the opposite. You have a NG grill but put LP orifices on it. This is simply "inconvenient". Because you will never get the output down to a normal "low or medium" level. But, at least it's not going to be a safety issue.
To my knowledge Weber is the only company who makes their grills this way. IDK why, but they're a royal PIA IMO. Because of it. About every other mfgr I know of makes a simple kit to convert between fuels. Some are so simple they only need one orifice change.
Weber could easily make conversions like this they choose not to