I have been riding on tubulars all my life and the the last 3 years I rode over 20 000 kms on the dirty roads of Brazil. For sure I am byassed, but I could not imagine to use the in general much heavier clincher combination.
Regarding your questions:
1. Weight
With a traditional clincher wheel is easily 100 to 300 gr heavier, than a similar tubular combination for a person with the same weight. Furthermore the lightest wheel combinations nowadays are of carbon. With the present carbon wheel trend for super light wheels you have no choice than to use tubulars. Clinchers cannot be used.
2. Mounting
Like with everything new, the first time can be tough. But I am sure, that mounting a new clincher (worse a tubeless clincher) will as tough or tougher, than glue your first tubular.
2. Toughness and security
The tubeless manufacturers are constantly developing new products and at present I am using Tufo tubulars from Checoslovakia. They have as well tubeless clinchers as tubulars and really very tough ones. I have training tubulars from them, with 2000 km and they are only half life.
A flat clincher means almost for sure a fall. With a flat tubular a fall is less probable, because the tubular looses air slower and does not get out of the rim as the clincher does in such cases.
3. Flats
When you have a flat tyre with a clincher, you will have to loosen the tyre of the rim and afterwards repare the tube or change the tube. This will take you at least 5 to 10 minutes.
With the Tufo system, when I have a flat, I stop, open the air valve of my tyre (clincher or tubeless), empty the tyre, screw in the Tufo sealant tube, inject approximately 10 to 15 gr. of sealant, unscrew the sealant tube, inflate the tyre, close the valve and ride on. This means, I do not have to change the tyre, whether tubeless or clincher, and even better it will be afterwards sealed for wholes of up to 3/4 of an inch. This is not an example it is practice. When I had to do it the first time it took me 5 minutes to find out things. Today it takes me the time to deinflate and inflate the tyre.
4. Price
They always say, that best tubulars are much more expensive than the clinchers. This is just a common opinion. The best Michelin clincher is as expensive as the best tubular from Conti, Veloflex or Tufo and they forget, that you have to include the rim sealant, the air tube, etc.
I hope I replied to your questions.
Best regards,