Are convertible full face helmets as safe as regular ones?
Yes, some convertible helmets can be as safe as Full Face Helmets.

The safety standard for the protection of a full face mountain bike helmet is ASTM F1952 - Standard Specification for Helmets Used for Downhill Mountain Bicycle Racing. This covers the chin bar and a higher level of testing than EN-1078 (such as a higher drop height 1.2 to 1.6). En-1078 is the standard for road and trail helmets.

Purchasing a helmet meet this standards means they will be as safe as a proper full face. Choosing a helmet that doesn't hold the standard doesn't mean that it isn't as safe, but it hasn't been tested to that level and doesn't offer the same assurance. The standard has two levels, one for the chin bar and one for the helmet.

In order to get the same protection as a full face you'd need to find a convertible helmet that held both such as the Giro Switchblade. If you are happy with a trail helmet with a certified chin bar but EN-1078 certified helmet then something similar to the Leatt Enduro. If you want chin protection and not concerned by the standard then something similar to the Bell 2R, which simply holds EN-1078.

The reason behind the Bell 2R not holding the standard is suspected to be vents on the chin bar. Bell have said that they test to internal standards, which may meet ASTM F1952, but they haven't certified the helmet against the standard.