Learn English – Formal/informal word for “something which is hard to deal with”

There isn’t one.

If you are going to write about this in a formal context (for example an academic article on how communication works in various forms of ‘limited’ media, such as texting), you would do one of two things:

1. Use an explanatory phrasing throughout, like “abbreviate words by removing vowels”, vel sim.

2. Use the term disemvowel with an explanatory note and perhaps ‘scare quotes’ the first time you use the word, and treat it as a context-specific technical term thereafter, thus removing its air of informality.

There are other verbs that have meanings similar to disemvowel, such as aphaeretise, syncopate, apocopate (for removing sounds from the beginning, middle, and end of words, respectively), synaeretise (for diphthongs turning into monophthongs), contract (a subset of syncopating only applied in multiverbal contexts, such as not being able to undergo syncope to n’t, but only when it appears after an inflected verb), etc.

These words are all more general, however, than disemvowel—they refer to the loss of any sound(s), whether consonant(s) or vowel(s)—and they are all based on the actual, spoken language, not the writing that secondarily represents spoken language. Disemvowelment, on the other hand, is an artificial by-product of specific (and very recent) types of written language: it is the removal of only one type of orthographical glyphs (vowels), and it is a removal that has no counterpart in the spoken language.

When vowels (and/or consonants) are syncopated in writing, it is to represent that they have been dropped in speech; when words are disemvowelled in writing, their pronunciation does not change: they are pronounced as though the removed vowels are still there.

The colloquial word, 'jaw-breaker', means "hard to pronounce":

jaw-breaker, n.
colloq.
1. A word hard to pronounce ....

Several other words meaning "difficult to pronounce" are in use:

break-teeth adj. (also break-tooth) difficult to pronounce.

'Break-tooth' has a distinguished pedigree, including use by Sir Walter Scott (1827).

crack-jaw, adj.
....
Fit to crack the jaws; difficult to pronounce. Also transf.

'Crack-jaw' has a history of use that is even more illustrious than 'break-tooth', including uses by Disreali (1827) and Beerbohm (1920).

'Cramp' has its own charm, but is likely to lift an eyebrow or two when used:

cramp, adj.
1. a. Difficult to make out, understand, or decipher; crabbed.
cramp word: a word difficult to pronounce or understand.

(All from the OED Online.)

The first three of these words will very likely be easy to understand, even for people not familiar with their use and meaning. The fourth may require explanation of its meaning in most company, unless it is used in the phrase "cramp word", which might make the need to explain its meaning less likely. None of them are designated as obsolete or rare in the OED.