I have some experience in designing gantry cranes in steel (although not timber) and I would say that for the sort of loads you are talking about this does seem a bit marginal, especially given that the material properties of a going length of timber are inherently less certain than those of structural steel sections.
A lot will depend on how the joints are made, again wood is that bit less predictable than welded or bolted joints in steel so you need extra margin of safety to accommodate this. But suffice to say that if it feels 'wobbly' this is not a good sign.
the situation in the second picture of your question is just bad. This imposes a huge bending load on the upright legs and makes the whole system inherently unstable. If you want to do this you need adjustable legs to keep the legs vertical and the beam horizontal. This sort of lean would not be acceptable for any sort of gantry crane, not matter how well constructed.
In this sort of design the likely failure points are the joints between the beam and the legs and buckling of the legs themselves.