Sounds like you want a lot of pins you won't have. Make sure you don't want many analog or PWM pins, and be mindful if you play with spi you lose the USI pins in one go. I found the RAM was a big limitation for using one with an infrared controller...amazing how fast you can chew it up.
Remember one of the legs is also the reset so it has limitations too. And of course there's no serial monitor for debug messages.
On the flip side I found using one with a pair of shift registers was a breeze for adding 16 digital outputs...but by the time you do that you could have bought an ATMega328 for the price and had more RAM...
And I've not strictly answered your question. Compared with a UNO you lose power regulation, USB, shields, the ability to have SPI & I2C at the same time, the onboard status LEDs...but I'm pretty sure you were asking to contrast the 2 processors when used standalone...
You will have fun, though in my short time playing with them I find I think of a project that suits ATTiny and code for it from the start rather than migrating a working Arduino project to the smaller chippies.
Cheers!
Geoff