Trip reports forum

I can only answer for Thailand - I visit quite a few times in wet season because things are more elcheapo than norma.

A typical wet season day has one or a few showers or storms, usually of short duration, separated by periods of sunshine/scattered cloud.
Fully overcast periods do occur, but because it is hot, can be great for the beach. Note you can sun-burn badly in such conditions unless the overcast is really thick. Prolonged fully overcast periods lasting several days are not all that common.
At least 30% of shower/storms are at night - if this is the only shower/storm that day, you are laughing.
Prolonged bad weather periods are rare, but can happen - from anecdotal reports, most often in Sept/Oct on the Andaman Coast and Oct/Nov in the southern Gulf. In such periods it can be extremely windy, with big seas and many boats staying in port.
All wet season months have several days with NO rain. Check links like travelfish http://www.travelfish.org/weather_fish.php for this. Sometimes you can get 3 or more days consecutively without rain, sometimes with little cloud. I got more than a week of the latter in my first November visit to Samui - November is supposed to be the worst month there.

BONUSES OF WET SEASON TRAVEL
Lots of vacancies, lower prices for accommodation, un-crowded transport.
Lovely cooling impact during and after a shower/storm. That special just-rained smell.
The forests and countryside gets lush and green, the waterfalls are no longer trickles, and "especially central, north and north east: nature isn't bone dry and comatose, and less dust or haze" - CHANCHAO.
Fabulous night-time electical shows.
Great cloud formations, fantastic sunsets/sunrises for photographers etc
Doesn’t get much more chilled than swinging in the hammock on your bungalow verandah with a bottle of Maekong as the drops from the late afternoon shower plop-plop off the trees onto the surrounding banana leaves.

DOWNSIDES OF WET SEASON TRAVEL
Some people don’t like the higher humidity.
Late wet season is mosquito season.
Some places close down. Note that this seems much less common now than 10 years ago. All the well visited locations seem to have plenty of places operating now. It’s also surprising how many less visited islands have at least one place open.
Some ferries shut down eg the Krabi -Jum-Lanta and the PP- Lanta. Some mainland travelers’ routes don’t run - eg as far as I know the Krabi - Khao Sok and Krabi- Khao Lak minibus services.
During the rare extended stormy/windy periods swell can get up and normal wet-season ferries/long tails can stop running. It is maybe not such a good idea to run too tight a schedule to catch that departing mainland flight.
Lots of rubbish is carted downstream by bank-full rivers and blown onto beaches during the wet season. Keen bungalow operators clean this up. Less keen don’t.
Storm water runoff can worsen diving and snorkeling visibility and can attract jellyfish in some locations.
In some areas prolonged windy periods lead to big surf/dangerous rip-currents/drownings - southern and south-western Phuket beaches and the main big Ko Chang beaches are noted dangerous areas during these periods. Be careful. I tend to look for beaches on lee shores during wet season for lack of rubbish and smooth sheltered conditons - Ko Samet, the southern Gulf islands, Ko Ngai, the chill east coast of Phi Phi are some which have these sheltered beaches.

I visited the countries you mentioned in June/July this year and had no weather-related problems.