How limited are Cantonese dictionaries?
It depends on what you mean by "Cantonese". The practical definition is Yuehai Yue, from Guangzhou down to Hong Kong and Macau.

Any dictionary will be limited. From a purely practical point of view, the scope of the lexicographer's project must be in some way worth the resource put in. Hence dictionaries limit their scope quite significantly.

In practice for Cantonese then, most dictionaries [as pointed out above] specify not 廣東話 but specifically 廣州話 - the bias towards the Yuehai 粵海 region is made explicitly. 

The Dominance of the Yuehai Prestige Dialect

This is (in essence) one of the definitions of a prestige dialect, and the Cantonese as spoken in Guangzhou 廣州 has always been the leading example of the Yue 粵語 branch of Chinese. The fact that people originating from Guangdong 廣東 and Guangxi 廣西 provinces were already using the speech of Guangzhou 廣州 as their lingua franca gave it a prestige (the province was divided into two in the late Song dynasty). This meant in the early Repulican period, a Teochew 潮州話 speaker, a Hakka 客家話 speaker and possibly a Hmong speaker would have used 廣州話 as their lingua franca, even if they were doing business in Nanning 南寧 in Guangxi. Across the Sinosphere of SE Asia, the older term 廣府話 became standard across both Yue and Min speakers.

The rising prestige of Hong Kong 香港 through the 20th century only reinforced the trend of Yuehai dominance. The Yue-speaking diaspora throughout the world had already adopted the speech of Guangzhou 廣州話 by the mid 20th century (the largest Yue-speaking group that didn't do so was the Taishan [Toisan/Hoisan] 台山 group in California), and with a greater influx from HK into the diaspora, the strength of the Yuehai group of Cantonese increased within the Cantonese community overseas, even as Mandarin made strong inroads in Cantonese speaking territories.

On the other Yue varieties

Hence, any interest in the other Yue varieties would be documented, not by mainstream dictionaries, but fairly specialised linguist's dictionaries and papers. The dialect of Bobai 博白, Guangxi Province, is well known for having one of the largest tone inventories in the world.

The divergence and diversity with Yue should not be underestimated. The perception as just a "rougher version" (lit.: 鄉音 village sound) of (Guangzhou / Yuehai) Cantonese is broadly untrue. Everything from tones, sound system, [synchronic] sound changes, even some grammatical constructions as well as vocbulary can be different. Hence you are right to say that a lot of words would not be found in a mainstream Cantonese dictionary. But we would need a few more dictionaries to made before this can be quantified.

One of the most salient and well documented examples is Toisanese, which maintains the Middle Chinese distinction between alveolar and palatal/retroflex sibilants, but recasts it as a /ɬ/ vs /s/ distinction, and has a toneme for making the plural pronouns (我 /ŋɔɪ˧/ for "I" to 哦 /ŋɔɪ˨˩/ for "we"). 

In the 現代漢語方言大詞典, only two Yue varieties are represented: Guangzhou (of course) and Dongguan. Wiktionary has some support for Toisanese.