What are disadvantages of using brand or trade mark name that is already registered?

Confusion. Once a brand is established people associate it with the products it sells or services it offers. Also, even if it is legal to register it in your own country, you could very well have problems should you ever want to branch out in the future. It's always best to try to come up with something unique and identifiable.

Complex question. Many factors come into play.

Best solution, use a different brand name. If you're working in a very different industry, likely this won't ever bite you.

That said, I'd suggest you get Legal Shield coverage in your jurisdiction of domicile (where you file your taxes).

They're a legal COOP based in Ada Oklahoma + are available now in many countries.

Here's a real example of how this works.

The last time I was sued (in the US), I paid my $26/month (likely more expensive now) + the person suing me had legal fees of $42,000 for the first month of the suit.

I then called up the person suing me + told them next month, their legal fees would double + I'd keep doubling it every month, they the lost every dime they'd ever made or ever would make.

Since your Legal Shield attorney is working for you, your attorney will know how to wreck havoc on anyone suing you, so likely anyone suing you will go bankrupt as a result.

Summary: You never know when you'll be sued, so get Legal Shield coverage... so... like me... every time you get lawsuit paperwork in the mail, you have to sit down because you're laughing so hard.

I'm a trademark attorney and have experience advising clients on issues such as this. If the name you're interested in using will be connected to different products and services than the mark registered in Japan, there may be very little risk of confusion. You'll likely have different customers/clients and different marketing regions.

Think of the name "Dove." We have "Dove" bar soap and "Dove" chocolate. Same name; however, the products are completely different and are protected in different trademark classes. There is no risk of confusion and the two are able to coexist.

One anticipated risk is the mark could ultimately be diluted (i.e. lose its strength and uniqueness), but that depends on a host of other factors. Please let me know if you have additional questions.

The mark you created, promoted by you and already known can be used by unfair competitors. Consequently, poor-quality, and inappropriate products (services) of one enterprise or entrepreneur may be distributed under the well-known TM of another enterprise, resulting in loss of trust and goodwill of the latter. In this case, you will need to take several appropriate actions to appeal the misuse of the designation (logo), in accordance with the current law, which will lead to unplanned financial expenses.
To prevent and avoid possible problems while using your logo, company name or trademark, you can register it under the law and receive a certificate for the mark for goods and services. This security document will give you the unique right to use and profit from this trademark.
The official registration of your mark for goods and (or) services has the following advantages:
a. Positioning the company and its goods/services on the market among competitors.
b. Legal protection of your activity name.
c. Increasing company prestige.
d. Guarantee the uniqueness of your mark.
e. Protecting your services/goods from illegal copying.
f. Protection against unfair competition.
g. You will not become a violator of the rights of the legal owner of a similar or analogous trademark, if you use this mark legally (at the same time you will be able to avoid the obligation to compensate for damages caused by illegal use);
h. You will be able to avoid extra expenses of fighting for your mark.
i. You will be able to sell your trademark or transfer the rights to use it for financial benefit.
j. After receiving the Certificate on the mark for goods and services, you have an opportunity to register a domain in the .UA zone.
k. Also, you will be able to affix the warning label (® and ™) in the name of your product.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath