Has anybody had a problem getting guardianship because there is no medical POA?

I was able to get guardianship despite never having been POA of any sort. I'm under the impression that's usually the case—that many are forced to seek guardianship precisely because there is not a (living) POA in place. I would not worry about being disqualified on those grounds.

Have your attorney file the petition as soon as possible, because courts are moving slowly due to COVID. It could be weeks between when it's filed and your hearing actually happens. Good luck to you! Start researching long term care options before you get the guardianship, so you'll be ready to act when you're legally able to. Expect every process to take twice as long as you anticipated, whether setting up the guardianship bank account or getting the VA to work with you. Get to know the closest notaries and places to fax documents, because you will use them many times. Unfortunately, those guardianship court papers are no magic bullet. People will still give you the run-around even after winning guardianship, but it does open doors, if just a crack.

Guardianship overrides any POA. You will need to prove that Uncle is not competent to handle his own affairs. Be careful having guardianship has responsibilities. You report to the State.

How close are you to this Uncle? If not close, you may want to let the State take over. Tell Adult family services that you cannot be responsible for your Uncle. That you have no POA and guardianship is too expensive for you. It can cost a lot, some places as much as 10k or more. It may be able to come out of Uncles's money if you win.

The process of getting Guardianship will take care of the issues with the VA as well as Family Services.
The process of getting Guardianship will be going to court, the Judge will listen to the facts and if no one else wants to assume Guardianship you will not have a problem. A letter will be sent to all relatives that he has and it will give them a time to respond by and let theem know of the court date. If anyone wants Guardianship they can appear.
Once the Guardianship has been granted you will have all legal rights to care for him and make decisions for him based on his best interest.
(The VA or doctors will not be able to respond to the attorney due to HIPAA regualtion until you have a right to be informed of personal information due to the privacy)

Go ahead with your attorney in seeking the guardianship. Request a visit with social services at VA first and let them know you will have court proceedings due to his current mental status, for guardianship. Ask if they can help in any way. They will be summoned by the court if needed. Meanwhile the sad truth is you may have to pay for evaluation of mental status outside of the VA system. I would follow the guidance of your attorney in this matter. It is complicated; so sorry.