As has already been mentioned, “dual hose” “portable” AC units do exist. So, we need to consider two separate questions: are single-hose ACs bad, and are portable ACs bad?
Single hose operation
For instance, the first point seems to be equally true for any A/C unit, even the ones that have an outdoor unit. The air has to come from somewhere, right?
Yes, but a conventional household window or "split" air conditioner has two independent air flow paths. Air is taken from inside, passed over the evaporator coil, and sent back into the room, colder and dryer. Separately, air is taken from outside, passed over the condenser coil, and sent back outside, warmer. (Or, exactly the same but warm inside and cold outside, for a reversible ("heat pump") system in heating mode.)
A dual-hose portable AC has the same refrigeration apparatus and the same separated air flow paths, but the outside air, inflow and outflow, passes through hoses instead of just grilles on the sides of the unit.
A single-hose portable AC just leaves off the outside air intake hose, so it is taking inside air and blowing it outside carrying heat.
The air that is being sent outside is coming from inside the room. Thus, some conditioned air is being wasted, and replaced with (warmer, wetter) unconditioned air coming in through building leaks and open doors. So, single-hose portable ACs are less efficient. (We can confirm this from the big-picture perspective of thermodynamics, which tells us that any time we take substances of two different temperatures and mix them (here, the cooled air and the leaked-in air), we are discarding the energy that went into creating that temperature difference.)
There's a secondary problem: The negative pressure extends to adjacent rooms (unless you have unusually airtight doors). As a consequence, running a portable AC in one room will tend to make other rooms warmer because they're experiencing the warming effect of pulling in more leaked outside air all over, but not as much the cooling effect of the AC's output (unless you have fans to blow it all around).
Neither of these is a problem in a dual-hose AC — theoretically. (In practice, users have found that some dual-hose units have poor construction such that the "outside air" side of the machine is not very airtight, and thus tends to pull in some inside air.)
Machinery inside the house
There are disadvantages of the portable form factor:
In a window or split air conditioner, the noise and waste heat of the compressor (and fan for the condenser coil) can be kept outside the building. In a portable AC, they are inside.
As the hot exhaust air (and somewhat less hot intake air, if applicable) pass through the hoses, the hoses heat up and pass some air back to the room. They may also have leaks at their ends, in cheap plastic poorly-sealed fittings. Thus, some heat that should have gone inside is transferred back into the room. (It is possible to make or buy a insulation wrap for the hose, which will reduce this effect.)
Both of these can be thought of as inefficiencies that are incidental rather than fundamental — if we put in lots more insulation and sealing against noise and heat, then it would be less noisy and more efficient. But then you'd have a much bulkier, more expensive unit.
Point by point
This section will reiterate what I said above, but in context of your questions.
I read an opinion somewhere that almost all mobile air-conditioning systems are "working against themselves".
True, because almost all mobile air-conditioning systems are single-hose units. (Why? I don't know. Probably because they are cheaper to make and look less bulky, and only recently did (USA) efficiency ratings start requiring accounting for the inefficiency due to the outside air brought in, if I remember correctly.)
An A/C whose motor and compressor is inside the house as opposed to being mounted on the outer wall is inherently "flawed". I am talking about an A/C with a single outgoing exhaust hose
The inherent flaw is in units with a single hose, but it is possible to have the compressor inside the house with a dual-hose system, which is still less efficient but not “flawed”.
- By pumping the hot air out, the A/C creates an under-pressure in the house which sucks in more hot air from outside
- The heat of the motor and compressor stay inside the house
This issue is mostly noise, not heat. The compressor can live in the exhaust air-flow path, so it is cooled by ordinary “fan blowing air over the equipment” and does not consume any of the heat-moving capacity of the refrigeration system per se — in this regard, portable ACs are the same as window or split ACs.
- It is impossible to engineer the compressor to be quiet enough for a bedroom
I don't care to comment on claims of impossibility, but I haven't heard of this problem being solved in practice.