ISA RAM cards are limited to the speed of the ISA bus, while (depending on chipset) the RAM is connected to the CPU over a much faster bus. If you're playing a game, then you are already using a lot of the available ISA bus bandwidth putting data into video memory to be shown on screen, so if you also have to spend more bus cycles retrieving that data from a memory card over the ISA bus first, then there's going to be a lot less bandwidth available to the video card. This results in a lower framerate, sometimes significantly lower depending on the game and how much data it has to retrieve from the memory card.
This is why ISA memory cards were popular for spreadsheets and other low framerate applications, but why they were never widely used amongst gamers.
Access to the memory is also often different. The PC can access (generally) 640 kB in real mode and going beyond that requires EMS or XMS, which are protocols the programmer must use to access the memory. If you're not a programmer then think of the difference as having something written in front of you (native memory) versus having to call someone to ask them for the information you need (EMS and XMS). Because of this added complexity, most games that will work on a 286 and earlier don't bother with this extra memory even if it's available. If they do, they generally use it for storing the sound effects and not much else. So if your card is configured to provide EMS or XMS then it's only of limited use, and if you have a PC with less than 640 kB of RAM and you use the card to top that up to 640 kB then you incur a speed penalty.
In other words, ISA RAM cards are aimed at either business software, or to bring a cut-down PC with missing memory up to spec, for situations where running a game slowly is better than not being able to run it at all.
Of course it's not all bad. Some games (like Monster Bash) require EMS for sound effects, so for a 286 if you're lucky the chipset will emulate EMS but otherwise an ISA RAM card is the only way to get digitised sound effects in this game. If you're using a slow hard drive from the era, then you can use the RAM card as a large ramdisk, to speed up the machine. So they do have some uses, they just have some limitations to be aware of as well.
Make sure you get the drivers and manual for the RAM card though, otherwise it will be next to impossible to get it to work. They tend to have very complex jumper settings to set all the memory addresses and without the driver you won't get EMS support.