If it has to be waterproof/outdoor rated, that's going to limit your choices. I think ARB has one such model. I don't know of any of the "cheapies" that do, but.... maybe?
I have (accumulated over the years) four Dometic portables. Three CF models and one CFX. The CF's all use the Danfoss/Secop compressor (gold standard). The CFX uses a "Dometic's own" compressor, but it seems to work fine (and claims to be able to go to a lower temp).
One thing to check on if it matters to you: In ye olden days (i.e. 6 years ago), this type of unit would always have a 110 volt option, and would automatically favor that if 110 were available. IOW, you could keep the 110 plugged in all the time, and then if it showed up as available (on the other end of the 110 cord at the outlet) the unit would switch over.
Conversely, if you had it running on 110 and for some reason shore power failed, it would seamlessly switch over to 12 volt and just keep running. I have that on all my units and it does come in handy sometimes.
On Dometic's newer units (model number has a Q in it), they removed that option. A problem was apparently accidentally discovered by owners (IIRC 110 could bleed into your 12 volt system), and instead of fixing it, they simply sent stickers to those owners and on newer units tell you not to ever have both plugged in at once. That takes away the "fail over" option, if it matters to you.
I have one of my Dometic CF units in the cockpit of my boat. I do have a bimini, but no side curtains. I have the made-by-dometic insulated cover for it. It has mesh sections over the compressor vents. Certainly not waterproof but a slight amount of rain will sort of "catch" on the mesh. I coudl also make some little deployable hoods I suppose, if need be.
Salt spray would probably be much harder on it (I've been in freshwater the past few years).
As was mentioned above, these units have much less insulation than a built in (and are difficult to add insulation to, unlike a built-in). Also, most of the mfgr insulated covers (which are nicely fitted) are very dark grey or black (??whyyy??). I keep a yellow throwable cushion on top of mine for that reason.
Another note is that most of this type of unit (even if they have a Danfoss/Secop compressor) add their own digital thermostat and some circuit boards where you control the temp. I have had those fail, along with the thermistors (sensors); but it's not hard to remove all that and add your own $20 digital control/sensor and controls (and they actually have more functionality in some ways).
I can't speak to the "knock off" compressors; but I have a friend with a Bouge RV (basic china compressor) unit and he's been happy for a couple of years. I will say my Danfoss (Dometic) units have lasted ten years so far, but they are lot more expensive and I would no longer buy a Dometic due to the way they handled the 110/12v failover issue.
Lastly, once you've decided to either spend more for a Danfoss type compressor or to go with a knock off type, I think as important is looking at the box features. Is it the shape you want? Is the lid easily reversible (keep in mind the compressor can't be moved and you need to provide good air flow to the intake/exhaust vents which are usually on either side of one end)? Is the liner leak-proof or full of seams? Do the lids require you to have an extra 5" on the hinge side in order to open fully (so like a recliner, no putting it near a wall)? If the hinges are on the short side, the lid will be tall so you need more height to open it. Etc. Etc.
