It's all about your point of view and how you want to phrase (sugarcoat) it.
The majority of 4/32 devices barely boots and even if they do performance is less than stellar except for (more or less) late Atheros based devices which works in some cases.
opkg and LuCI do run unreliably and will cause memory exhaustion, a barebone install will run but you'll have very little memory free which means limited amount of clients and connections. As most devices are quite old and/or aimed for the budget market the SoC is usually quite slow when means you can easily overload the CPU which will also cause unreliable behaviour (reboots, etc).
You can still use such devices in specific applications but I wouldn't recommend such as device in general as you will most likely trigger something mentioned above .
In general I think @jeff is on the right track, I would probably mention that armv7 (or better) is a preferably to performance and overall "support" as MIPS SoCs are slowly being deprecated.