TLDR: No, spot colours are not more limited than CMYK, and they aren't treated the same because they are different printing processes.
Process printing (or CMYK printing, also sometimes called full-colour printing) uses separate inks. They aren't mixed.
The inks are layered onto the paper, usually using halftone dots to build up a pattern of dots to simulate a solid colour when viewed at normal viewing distances, or often as not for reproducing full-colour illustrations or photographs. In offset lithographic printing, this usually means using a press with four print heads, and four printing plates, one for each colour, or sometimes using one or two heads with multiple passes through a printing press.
A typical 4 colour process offset lithographic printing press, showing the four colour heads (CMYK), and ink ducts, which supply ink to the rollers, and subsequently to the 4 printing plates, and the image is transferred to a rubber blanket, and then onto a sheet of paper under compression between the blanket and a steel cylinder.
A spot colour is printed using one single ink, often made by physically mixing different base colour inks to a formula from a Pantone guide book - a bit like the same way you'd mix some base paint colours to get a new colour.
Mixing a spot colour ink
Each spot colour is printed using only one printing head, and one plate, and typically with solid areas of ink, although it's possible to use them with halftones too. Spot colours have a wider gamut than CMYK colours, and there are spot colours which have no equivalent in CMYK - for example, metallic colours, or fluorescent colours, or inks with pure pigments such Reflex Blue, which is notoriously difficult to reproduce in CMYK.
Sometimes very large printing presses have more than 4 print heads, allowing the extra ones to be used for spot colours. Multiple spot colours are possible, and spot colours can also be layered on paper to create different colours.
You will likely need to use a magnifying glass to see it, but this is what a printed process colour looks like compared to a solid spot colour close up.