For one, the entire alphabet is arranged in a different order and some of our modern letters have NO Hebrew equivalents. Thus, based on the Hebrew alphabet, the only letters for which we have numerical values are the following:

               1    2     3    4     5     6    7     8
               ----------------------------------------
               A    B    G    D    H    V    Z     P
               Y    K    L    M    N    W
               Q    R    S    T

Obviously, a modern numerologist wouldn't get very far with this
table.
In order to compensate for the missing letters in the Hebrew system, most modern textbooks on numerology 'fill in' the missing letters by 'borrowing' numerical values from the Greek alphabet, thus mixing cultural symbols in an eclectic approach that is not entirely convincing.
Another problem is the exclusion of the number 9 from the table -- which modern textbooks often 'explain' by saying that the Hebrews did not use the number 9, since it was a 'sacred' and 'mystical' number.
The real truth, however, is far less esoteric. The fact is, the Hebrew alphabet DID have letters with the numerical value of 9 -- the letters Teth and Sade. But, since Teth and Sade do not have
equivalents in our modern English alphabet, the 9 value must be left out.
And finally, it is once again difficult to see any intrinsic relationship between a Hebrew letter and the number it represents. Why should one symbol stand for 1, or another for 2, or yet another for 3, and so on? The whole superstructure seems somewhat shaky.

But let us now turn our attention to a Celtic alphabetic system called the 'Ogham'.
This alphabet is written by making a number of short strokes (from 1 to 5) below, above, or through a 'base line' (which in practice tended to be the edge of a standing stone). Thus, A, O, U, E, and I would be written, respectively:

               ---/----//----///----////----/////---

Of course, in this system it is easy to see how a letter becomes associated with a number, since the numerical value of each letter is implicit.
Thus, A=1, O=2, U=3, E=4, and I=5. (It is true there is much disagreement and confusion among modern scholars as to how the Ogham alphabet should be rendered. Further, a number of different Oghams seem to have been employed at various times by different Celtic cultures. But this confusion usually centers on whether the strokes should be above, below, or through the base line -- not on the number of strokes used.