The quick answer would be yes, in most cases… but not always! Let’s dive in a little deeper.
BEST PRACTICE: The plumbing industry suggests that when burying PEX piping, an unbroken
line of PEX is ideal, and plastic PEX fittings should be used in areas known to have aggressive water or soil when a fitting is necessary. Stainless steel clamps, pinch clamps or rings should be used rather than copper crimp rings due to their increased corrosion resistance. This is another example where the industry endorses the use of stainless in direct burial applications.
ADHERE TO ALL APPLICABE CODES: The AHJ (Authority Having Jurisdiction) always has the final say on whether a given product and/or material is suitable for the specific application or project in question. In addition, one must make sure to adhere to all the applicable national and/or local plumbing and building codes for the area.
Special requirements for underground installation: It is important that if a SSPEX or SSCEP
series fitting is used that it is properly wrapped when making an underground installation. Wrapping with a quality silicone tape will help the connection last by providing an impervious barrier between the fitting and the soil. Wrapping ALL underground connections provides protection when there are unknown and/or adverse soil contaminants which will accelerate the rate of corrosion of even corrosion resistant 304 stainless steel fittings. There are areas were soil conditions have a pH value less than 4.5 and there is a presence of chlorides and stray currents which could have adverse effects on stainless steel that must be considered.
WARRANTY: It is very important to wrap all underground fittings for warranty protection.
Failure to properly wrap SS PEX fittings will void the warranty! What is Boshart Industries warranty policy?
While one solution will not fit all applications, the information below will be useful in making an educated decision on which material is best suited for direct burial in your geographic location.
STAINLESS: The use of Stainless Steel has grown in popularity since the introduction of the SDWA (Safe Drinking Water Act) in 2014 due the extremely high cost of No-Lead “Bronze” alloys. 304 SS Insert Fittings for Polyethylene (PE) pipe have been successfully used to install buried water service lines with great success.
304 Stainless PEX / CEP fittings are immune to dezincification corrosion and are gaining in
popularity due to their corrosion resistance. However, there are areas were soil conditions have a pH value less than 4.5 and there is a presence of chlorides and stray currents. These could have adverse effects on stainless steel that need to be considered.
PLASTIC: Polysulfone (PSU or PPSU) PEX / CEP fittings are highly durable, corrosion-resistant plumbing fittings. They are ideal for hot and cold potable water distribution, specifically resisting aggressive water, lime buildup, and chemicals that can degrade traditional brass. One must consider that plastic fittings will not have the same high impact resistance of stainless fittings so more care must be taken when back filling.
BRONZE: Bronze material would be an excellent choice for direct burial. However, there are not bronze PEX or CEP on the market to the best of my knowledge.
BRASS: Brass (copper / zinc alloys known as yellow brasses are the norm for PEX fittings. It is
important that you do not confuse brass alloys with the superior bronze alloys. One must be sure of the alloy copper and zinc content when selecting PEX / CEP fittings for direct 
burial applications as there are lower cost brass insert sold. Brass fittings may last a long time in ideal water / soil conditions. However brass alloys having more than 15% zinc content
(yellow brasses) are subject to dezincification and are not recommend for a direct burial applications. If brass is selected, make sure to use dezincification “resistant” brass (DZR Brass).
COPPER: There are copper PEX fittings available, copper is often used for direct burial, however copper can also be aggressively attacked and corroded by certain
types of soil. While copper generally has excellent resistance to soil corrosion, soils containing high concentrations of chlorides, sulfates, or ammonia compounds can cause rapid deterioration. Wrapping copper fittings with a protective insulation designed to withstand moisture, soil acidity, is also recommended.
Additional Resources:
https://www.ibexaustralia.com.au/resources/stainless-steel-underground
If salts such as sulphates and chlorides are present in the soil and is known to be aggressive, the pH value of the soil in which the stainless steel is buried plays a major role in corrosion and degradation. More acidic soils may represent a serious risk to common construction materials including some stainless-steel grades. Soils acidity is produced by mineral leaching, decomposition of acidic plants, industrial wastes, acid rain and certain forms of micro-biological activity. If the pH level is greater than 4.5, corrosion of buried stainless steel due to acidity is unlikely. As a rule of thumb, well-drained soils are generally found to be less corrosive than poorly drained ones. Backfilling pipe ditches with sand provides reliable long-term protection in corrosive soils such as silt.
https://www.assda.asn.au/publications/technical-faqs/faq-7-guidelines-for-use-of-stainless-steel-underground
https://www.worldstainless.org/applications/architecture-building-and-construction-applications/
CONCLUSION:
As mentioned above 304 SS is used successfully in many direct burial applications, however it is not suitable for every direct burial application due to aggressive water and soil conditions in some areas. These factors must be taken into consideration when making product selection decisions. Your plumber and/or local water works professionals will have the best knowledge of local soil and water conditions to provide insight as to which materials are best to use in the geographic area. In addition, they should be aware of any local requirements. It is always a good idea to check with local authorities before work begins - some codes specify the type of pipe material allowed based on the length of the service line, the incoming pressure, or other factors.
We trust this information has been helpful to you in making an educated decision on material selection.