They are really for packaging wine. I have a 200L one that I ferment sugar washes in, and it works well as a fermenter.
The steel is very thin, so you don't want to knock them around alot.
The lid seals with what is essentially a bicycle tube. The Stainless part of the lid has an OD smaller than the ID of the tank. In between that space, you pump up a tupe with air to make a seal that has an adjustable height.
The reason for this is to keep the O2 away from your wine, when you are dispensing it. It is an alternative to bottling.
Because it is thin gauge, direct fire would not be the greatest idea underneath one of these. You could compensate by placing it on a secure plate that was direct fired, but that really would be only compensating.
They cannot be pressurized, so have no real use for beer packaging.
If you want a really nice primary fermenter, and this item is in the volume range you desire, then I highly recommend.
You could put electric elements in them to avoid the direct heat issue, but they will still be pretty flimsy as brewery kettles.