Sometimes it's okay to start from the bottom and work up.
In the absence of access to original records, what are the risks of relying upon Tanguay's Dictionnaire?
Approach every source knowing the information therein is subject to error, oversight and omission.
"Know thy source." The Dictionnaire is a [massive] comprehensive scholarly compilation, thus it generally provides secondary information. The entries are unsourced; if I were citing information from the Dictionnaire, the citation would include a notation that, "no further reference was given."
I assume that "absence of access to the original records," means the genealogist/family historian has not sought out underlying records and information (rather than unable to access). As with any search that is not "exhaustive," the risk is that the researcher has missed something; their reason and logic will not have been based on a full range of information/evidence.
The Dictionnaire is a key, go-to, gotta have it resource for genealogists. Those I know who work regularly with the material can't imagine working without it.
Enumeration of errors/speculation in Dictionnaire.
The Wikipedia article about the author ("Cyprien Tanguay") cites four associated works which supplement, extend or correct the original work. Three of these follow. (I did my best to locate associated entries in WorldCat. There seem to be multiple versions with different titles; you will want to investigate these further.)
J. Arthur Leboeuf and Reverend Cyprien Tanguay, Complement au dictionnaire genealogique tanguay (1975; reprint, Montreal: Publications de la Societe Genealogifque Canadienne-Francaise, 1957). Wikipedia reports this work identifies thousands of errors and omissions.
Cyprien Tanguay, A travers les registres (Montréal: Librairie Saint-Joseph, Cadieux & Derome, 1886). From Wikipeda, this is sometimes called a "bonus volume"; 300 pp. "hundreds of facts ... historically related to ancestors ... collected by Tanguay at the time of records perusal." WorldCat lists editions published as recently as 1978.
Fr. Archange Godbout, Origine des familles canadiennes-françaises (Montréal: Éditions Élysée, 1979).
Will my research to contain errors if my sole source for facts is Tanguay?
His information is subject to error and so is ours. "Facts" are the result of reason and logic. Someone who relies solely on Tanguay is probably missing out on most of the fun and reward. At least for me, there is great reward in the opportunity to solve a problem by applying my own reason and logic based on a mass of diverse sources and information.
Do experts still consider his work to be sound?
Anytime a work is held in such regard by researchers, it will be subject to criticism. Any source can contain errors; it is a bit over the top to assume a work so extensive does not contain errors, etc. A line from the Wikipedia article struck me; paraphrased here, "Tanguay's work has been supplemented by others, but it has never been supplanted."
A biography about Tanguay appears in the Dictionary of Canadian Biography Online. The article provides some background on what became his lifelong mission to document so many families, and it says, "posterity has readily granted him another title: father of genealogical studies in French Canada."