I've had 3 front add-ons, 1 NV and 2 thermal, so i can't be blamed for speaking from ignorance of the subject
In all 3 cases i found the following:
1. The weight forward of the scope upsets the balance of the rifle
2. You need long arms to reach the focus adjust and other controls
3. The scope needs to have a low base magnification anything much above x3 and you won't see the icons and the field of view will be narrow
4. Maximum magnification depends on how much pixelation of the display you can cope with, but typically, it won't be more than x8 (the add-on itself is x1 magnification)
5. With the correct lens adaptor, most thermal add-ons also work as thermal spotters so that when a target is spotted the lens adaptor is removed and the add-on fitted to the scope. Depending on the model, that process can be fast enough to actually get a shot, or so slow that the target will be in the next county before you're ready to shoot
5. Retaining zero when re-fitted to the rifle, and even worse, trying to retain zeroes when used on multiple rifles.
A lot of the zeroing problems are down to the quality of the adaptors supplied with the add-ons because they are sold to cover a range of objective bell diameters.
Pulsar use plastic spacer rings to fill the gap between the adaptor and the objective bell, while HIK use sticky backed neoprene strips for the same purpose. Rusan make adaptors which are sized to fit specific objective bell diameters, but they add another £180 to the price
Front add-ons sound like an attractive proposition but in practice few users actually get on with them.
Front add-ons only exist to get around the hunting laws in some countries where NV and thermal targetting devices (ie they have a reticle) are banned, but observation devices (i,e they don't have a reticle) are allowed. Using the scope reticle to aim with keeps things legal whilst giving some level of NV or thermal capability
NV and thermal scopes are legal in the UK so we don't need this work around and can use dedicated scopes which are, all round, far better suited to the task
Cheers
Bruce